The Internet is popular, all right. But is it a less dynamic force than we've all been led to believe?

To be sure, there are those who contend that even the estimates of 20 million to 30 million Internet users are too low, if one counts the users of smaller public and private computer networks, like America Online and Prodigy, that are linked to the Internet mainly to exchange electronic mail. But such links do not allow users to employ some of the most significant features of the Internet, including some of the most impressive ones for conducting electronic commerce on the network, transferring and publishing files, and initiating complex information searches.

"The important variable is what the usage is, rather than what the numbers are," said Gary Chapman, coordinator of the 21st Century Project, a science and technology policy study group at the L.B.J. School of Public Affairs in Austin. "If it's just E-mail, that would have some certain kinds of implications. You could surmise there were a large number of people who were exploring the Net and not getting their noses very far into the tent." 'Fire Walls' Disconcerting

And for businesses and policy makers who are planning for a digital future where the Internet delivers tens of millions of interactive users freely sending and receiving all sorts of information over the Internet, the rise of "fire walls," or security barriers, is a significant impediment. Mike Schwartz, associate professor of computer science at the University of Colorado at Boulder, said a 1992 survey of the Internet found that an estimated 35 percent of all Internet computers were barricaded behind fire walls.

"If you start looking at Internet services that require interactivity, things like billing or accounting across the Net, there might be nicer ways to do it if you could directly connect to people," Mr. Schwartz said. But if more and more Internet users erect one-way barriers, he added, "the services you can use are more limited."

Also, Internet surveyors agree, there is no way to know how many computers are truly active on the Internet -- as opposed to ones that are out of service, are turned on only infrequently, or exist only as an address with no corresponding machine. When attempts are made to contact a sampling of computers on the network, typically fewer than 30 percent respond.

Despite his deflationary data, Mr. Quarterman still predicts that the Internet will eventually achieve and surpass the size now widely credited to it. But the future isn't here yet, he says. Opportunity Knocking