Yes, biologists have identified about 1.4 million species. There are surely other species not yet identified and cataloged, but estimates vary widely on how many. I've seen estimates for total number of species ranging from less than 2 million to over 100 million.

But the majority of species are microscopic. By definition, microscopic creatures would not take up any measurable amount of space on the ark. They would have just been carried along in or on the larger creatures.

The International Union for Conservation of Nature -- no indication they're a Christian or creationist group, they're environmentalists -- estimates there are 1.3 million non-microscopic creatures in the world. You can get similar numbers in many places. Anyway, 1.3 million, still a lot. But 1,000,000 of those are insects and another 102,000 are arachnids, which don't take much space. 31,000 are fish. The salt-water fish, at least, would not have had to be carried on the ark: Noah didn't need to build fish tanks. Another 85,000 are mollusks, most of which are sea creatures. The only creatures Noah would have had to worry about are what's left: 5,490 species of mammals, 9,998 birds, 9,084 reptiles, and 6,433 amphibians. That makes 31,005 species. (See International Union for Conservation of Nature Red List, Gland, Switzerland, 2010, Table 1.) (And by the way, I'm just using them as a convenient source. You can find similar numbers from other sources.)

As others have noted, Genesis says Noah brought 2 of each "kind", not 2 of each "species". A baramin (kind) is usually broader than a species. So 31,000 is pretty much the upper limit.

So Noah would have had to bring 31,005 x 2 = 62,010 animals. (Okay, a little more for the creatures that he brought 7 pairs.) There are and were certainly some large animals in the world --hippos and elephants and allosauruses and so forth -- but most species are much smaller than this. The average animal weighs about 100 grams, about the size of a large rat.

John Woodmorape wrote a book called "Noah's Ark: A Feasibility Study" in which he devotes considerable space to calculations on how much space would be required. He comes up with a total of 15,754 animals. (i.e. he's counting baramin and not species.) He then estimates space requirements by using the amount of space given to animals in laboratories and factory farms. He makes the argument -- a valid one, I think -- that this is a good "middle number". Using the space given when transporting animals in trucks, train, or plains would be too small: such trips tend to be short, so the animals don't need a lot of room. But using space given in zoos would be too large: Zoos are meant to be comforable, and the environments are as much to entertain the visitors as to be pleasant for the animals. The Ark voyage was not a pleasure cruise. So anyway, using figures for space in labs and factory farms, he came up with a total space requirement of 4,300 square meters. That is less than half the floor space on the ark if built to the dimensions in Genesis. That's a high number, because it assumes all cages sit on the floor, but surely Noah could have stacked the cages of the smaller animals.

Woodmorape goes on to calculate that food for the voyage would have taken another 6 to 12% of the available space. If they had to bring along enough water for the entire voyage, that's another 9%. (In real life they could likely have captured rain water to meet at least some of their needs.)

So, as I say in my book, all the animals plus the food and water would have taken up about 2 of the ark's 3 decks. That leaves the other deck for quarters for Noah and his family, room for extinct animals unknown to us today, the dance floor, casino, and karaoke bar.