David Letterman--the gap-toothed, goofy, irreverent host of ''Late Night with David Letterman''--has help. It`s not that the man isn`t talented. It`s just that there are others aiding the cause. In Letterman`s case, help comes by the dozen. Eleven men, and a woman make ''Late Night'' irreverently succeed. Five came out of Harvard, three are Jewish, all but one were born after World War II and strangely, most belong to the National Rifle Association. All those ''Late Night'' pranks, gags, photo captions, skit notions, stylized props and telephone calls--except for the viewer mail--get written by these people, whose median age is 28. And the writers aren`t as ''zany'' as you might think. Two by two they share offices, jotting pages of ideas, creating and discussing future creations, joking around--they`re quite funny by nature--and commenting sarcastically on the state of the world. Every now and then a brilliant scene arises from the heap. It was Feb. 1, 1982, when NBC launched ''Late Night with David Letterman'' and gave Fred Silverman`s wonder boy another shot at television greatness. In 1981, the network canceled Letterman`s morning show, ''The David Letterman Show,'' after four months of poor ratings. ''Late Night'' (11:30 p.m. Monday-Thursday on NBC-Ch. 5) came on with the notion that television isn`t art; that Dave and his crew could, and would, do anything feasible to get grins. ''The only rule is that everything we do has to fit Dave`s style--all things he can create off of,'' says Merrill Markoe, a writer on the show (formerly the head writer), who now oversees most of what occurs behind the scenes. Though he laughs that it`s only network TV, head writer Steve O`Donnell says the truth is that members of Dave Dozen don`t hold television near or dear to them. ''We sit around in meetings and try to think of what will work. We never know what`s going to happen,'' Markoe says. O`Donnell concurs: ''All we do is make sure America has some entertainment late at night.'' Their goal, he says, is to ''make sure we take cliches in different directions.'' Rather than, say, Johnny Carson`s ''Mighty Carson Arts Players,'' you see ''A Brush with Greatness,'' where an audience member retells the ''experience'' of meeting a celebrity. Or ''Stupid Pet Tricks,'' a popular mainstay. Says Markoe: ''We didn`t plan for that to be a regular thing, just like we don`t know if most things are going to work out.'' But ''Stupid Pet Tricks'' originated on the morning show, as did ''Viewer Mail,'' ''Small Town News'' and silly photos from vacations. But it`s not only expected segments that make the show`s potpourri of entertainment attractive. Letterman also has guests--sports, TV and movie stars as well as authors, comedians, musicians and just plain folks. The writers` only goal is to buff up Letterman`s cynical, wink-wink, everything`s-a-joke manner. ''At the core,'' O`Donnell says, ''we all share Dave`s sensibility. We aren`t Letterman clones, but we are a lot like him (in a humor sense).'' Sometimes ''Late Night'' appears self-written. ''That`s good,'' O`Donnell says. ''We work to hide the fact that it`s assembled, and make it appear effortless (for Dave).'' The trick, then, is ''Don`t be strict to the clock.'' Or, if something has turned into a major ad-lib, they leave it alone. So Letterman`s a lucky comedian, having such dedicated admirers to see he doesn`t make a fool of himself in front of 4 million viewers each night. ''We just aren`t the cliche, and neither is he, of what television is all about,'' O`Donnell says. ''There`s this sort of robust metabolism--all right, not every day. Our biorhythms aren`t always in sync . . . . This group, we`re like family--really. We`re not highly sensitive, not egotistical, and we share Dave`s regular guyness in our attitudes and the way we work. ''We spend our days trying to do things that are different, assuring we don`t become staid. Hey, we aren`t supposed to be assembly line TV. This isn`t `Alice!` '' MERRILL MARKOE The only female on the staff says she was ''one of those 1960s art student types--a cross between a radical and an idiot.'' At 37, Markoe holds some distinctions: an Emmy for Letterman`s morning show, several TV scriptwriting and producing credits, the editing on an upcoming ''Late Night'' book, and being Letterman`s love interest for the last seven years. She jokes about going on the show with ''Staff Vacation Photos,'' having to coyly describe ''my boyfriend without saying who it is. It`ll get cut.'' Markoe, started on the show as head writer, which was ''a silly situation. Then, I seemed the logical choice. I had been trained to reflect his comedy. But you don`t want your boyfriend making all the decisions about your work. I was in daily battle with him.'' Happily, they let others grapple with the chore. Now, ''Steve (O`Donnell) handles him so well. We don`t even talk about the show anymore.'' When asked why more women don`t write for ''Late Night,'' she claims few have applied; the rejection rate is high. ''I think women comedy writers take jobs with better situations than this.'' As for the absence of black submissions, Markoe says, ''Blacks writing comedy usually perform their own material. ''I honestly don`t think people turn Dave on to see ethnic humor,'' she says. ''This show is his view of things; you`re not going to get ethnic humor from Dave. Some other guy should have the chance to do that show.'' Today Markoe handles the remote film clips--and specializes in the pet angles--working as ''one of the guys'' while waiting for the chips to fall on another network project. ''It`s much better this way. I enjoy having a place to go. I have the best opportunity to do so much.'' STEVE O`DONNELL Steve O`Donnell, responsible for smoothing all egos in the ''Late Night'' studios also must make sure the wrong skits aren`t aired. The 33-year-old head writer for two of his three years at Letterman`s says the ''biggest frustration'' all the writers share is ''working on something for a while, it ranges from a day to a couple of weeks,'' and seeing it miss the mark. ''There`s a tremendous attrition rate for these things,'' says O`Donnell. Ideas, he says, are like North Koreans in trenches waiting to be shot down. O`Donnell and company create four shows weekly, sometimes more for future dates, writing shtick for someone he refers to as ''a walking entertainment center.'' ''Dave is funny,'' he says with a certain adamance. ''We don`t have to work, work, work to make him seem that way. He makes it easy: Dave is well, a spiritual perfectionist.''

Words of wisdom emitted from a former Ohioan who studied history at Harvard, edited the National Lampoon and came to New York in search of ''work.'' ''I had a job in the library at the Museum of Broadcasting.'' He hadn`t planned on being head writer to the ''Late Night'' crew. ''I saw the morning show and felt a powerful attraction for the work.'' Inspiration hit him and he applied for a night spot. For now, he is pleased with his position: ''I see network TV as a big farmland and writers are like farmers. No, scratch that.'' O`Donnell says and flashes a wry grin. CHRIS ELLIOTT In an effort to be mischievous--no one gets away easily here--all those interviewed mention how Chris Elliott, the only ''actor'' in the group, scares them. He`s weird, they said. ''I`ve heard that before,'' the soft-spoken Elliott says. ''I`m much meeker than people think I am. I`m not really an actor,'' Elliott says, ''because I can`t audition with a bunch of guys wearing leg warmers.'' Calling himself ''lucky to be on TV at all,'' Elliott, the 25-year-old son of comedian Bob (Bob & Ray) Elliott, says the characters he portrays on the show--''The Conspiracy Guy,'' ''The Panicky Guy,'' et al--are ''sort of stupid'' bits written with partner Matt Wickline. ''They are very insignificant skits,'' which he qualifies as being ''more talk about the character than anything else. The opening lasts longer than the actual piece.'' He was a bit-player before being hired to create. ''There was a shortage of writers then,'' he says. ''Yeah, people laugh at my characters, but I`m sort of the running joke on the show. ''My `guys` have become these living cartoons--I always look like someone trying to get on the show.'' Elliott is, however, responsible for half the work on ''Custom Made Show,'' a gem that garnered an Emmy, which means his place is secure: ''I used to come up with big ideas for shows and still have some input in most things that go on . . . . I would certainly be happy to step down and just do a walk- on once a week. Writing and performing is great, but I could do either wholeheartedly,'' he says, flashing a grin. MATT WICKLINE At age 23, Ohio-born Wickline is not aware of any great envy toward his job. ''My friends are not the jealous type,'' he says deadpan. Most of his ideas run along those blunt lines. On burnout: ''I wonder about people who run out of ideas. I`ve been here for two years . . . . I have never run out of jokes.'' On writing teams: ''Chris (Elliott) and I are both funny--we rub off each other.'' Speaking in definitives, Wickline says he never fathomed a career in comedy until Letterman plucked him from an NBC internship. ''Nothing made me think I could write comedy. I had gotten out of school and didn`t have any experience. I just wanted a job that would keep me in New York.'' He came on when staff left en masse to write NBC`s superbomb, ''The New Show.'' He stayed. ''I`d say we have the best of both worlds. We can write what we want, within limits, and really get along. Hey, we disagree. Nobody ever gets into, `You jerk--you stole my idea.` We don`t scream; we sulk when things don`t go right. But nobody really cares. Why should we? It`s just a TV show.'' JOE TOPLYN A ''Late Night'' writer with an MBA from Harvard is indeed an anomaly. ''I suppose I am not putting it to much use,'' says the 31-year-old Toplyn about his facility with numbers. Formerly a marketing whiz for General Foods who touted dog food, then ''somebody`s personal MBA'' at Columbia Pictures, he one day noticed his humorous tendencies. ''I wanted to be in the entertainment business and I couldn`t stand being part of a large corporation.'' NBC only gives us renewable, temporary I.D.`s. We`re part of a small structure.'' He says he now is satisfied with his work. ''We are producing something different from anything else found on television. Where else will you see the host take a bullhorn and shout from the window, `You! Want to be on television?` '' Sitting behind a meticulous desk, the serious Toplyn identifies his biggest accomplishment to date as ''getting Dave to play tic tac toe (over the phone) with a Cuban when they were boycotting the Olympics. You couldn`t imagine the logistics involved in that. It`s incredibly difficult to get a phone connection with Cuba, and we had to find somebody who speaks English!'' SANDY FRANK According to Frank, he and sometime writing partner Joe Toplyn ''have the most wasted degrees'' in the office. Frank, 31, graduated from Harvard Law School and practiced law for two years. ''Let`s just say I wasn`t happy,'' he says. ''There wasn`t room for humor in a law firm. It was hard for me to relax with the partners who controlled my destiny.'' A writer for Harvard Lampoon, Frank tried for the better-paying ''Letterman'' job because he felt he was ''extremely well-suited for the job. I admit I had the easiest time you could possibly in switching jobs.'' He speaks in a droll, quiet manner and laughs when recalling how two years ago he had to ''unlearn normal office procedure'' and stop dressing up. He pauses, reflecting on the ''Late Night'' challenge faced daily. ''The only problem here,'' he says, ''is that shows by their nature have a certain character, and sometimes you`d like to do something that just doesn`t fit. You wish it could.'' Sketches, for one, are rare. ''Dave doesn`t have characters. I might want to do something like you`d see on `Saturday Night Live.` You`ve got a constant struggle because you can`t do it.'' GERARD MULLIGAN ''Burned out'' is how 40-year-old Mulligan sees himself. Though obviously in his prime, the dry-witted former history teacher explains: ''Here they don`t fire you when you`re finished, they give you some sort of executive position.'' Except this executive candidate continues to dish it out. Mulligan has been with Letterman since the early days--the ''Letterman'' morning show. Today the oldest of the writing crew stays useful by ''jotting down the things that make me laugh. For instance, helpless people being injured.'' He`s not serious. ''No, I guess not. The entertainment business, that`s entertaining. This is the most self-congratulatory field in the world! ''The basic philosophy of my comedy is to take the smallest thing and blow it completely out of proportion,'' says the self-termed ''vet.'' ''A new desk for Dave is monumental.'' Mulligan also does a lot of Dave`s ''Late Night'' opening remarks. ''By now I know exactly what Dave will and won`t do,'' he says. The bearded, heavy-set funnyman says his younger counterparts do not look up to him. ''Aw, they feel sorry for me,'' he says. ''Look, I`m sure they all wonder why I haven`t burned out. But plumbers work for 40 years without getting burned out!''