Well, we could exclude marine animals and breeds dog/cat/other domesticated species.
How many species can fit on the ark?
How big should the ark be to hold all the land creatures?
I would recommend talk.origins for a scientific discussion of this and other creationism-related issues. They provide comprehensive links to Creationist sites if you want to examine the claims of the opposition.
The size of the Ark is given in Genesis 6:15 as 300 x 50 x 30 cubits, or 450’ x 75’ x 45’.
Riiiiiiiiiight.
What’s a cubit.
Young people these days don’t even know what Candid Camera is. 
(Sorry for going OT, but he started it! Should this be in GD, or even CS considering where it’s going?)
The correct answer is “all of them.” Duh! 
Well, are you just looking for how many you can fit on there, or are you also accounting for the room that would be required to bring food enough for all the animals? Also, why aren’t any plant species being brought with to save them from destruction?
Reread Msg#2.
Multiply 300 cubits by 1.5 and get 450 feet.
Therefore 1 cubit = 1.5 feet.
** Cubit **
.
Don’t worry, Telemark. I got it.
“I’m gonna make it rain for a hundred days and a hundred nights.”
“Hey, look. Just make it rain for forty days and nights, and wait for the sewers to back up.”
“Riiiiight.”
In all fairness, there is a certain amount of vagueness in relation to the exact length of a cubit. According to Merriam-Webster it is “usually” 1.5 feet.
Except for the unicorn …
Isn’t there a line of thought that says that Noah didn’t get every species, but every genus? And how many of each creature did he take, anyway – two, or seven? Bible says both, so why have we settled on two, in the common understanding?
What is that wooshing sound I keep hearing? Hmm… maybe this will help.
http://www.clown-ministry.com/skits/Noah-skit.html
In regards to maximizing the number of species, do we have to keep the familiar creatures like cats and rats and elephants? Because they take up an awful lot of space. On the other hand, if we’re starting with insects and similar tinies, we could probably get most animal species.
There was green alligators and long-necked geese, some humpedty-back camels and some chimpanzees…
Seriously, some disclaimers:
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Only a minority of Christians and some Orthodox Jews take the story literally; most understand it to be fabulistic, told for theological reasons we need not get into in this GQ thread.
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Those who do take it literally note that Noah took two of every unclean “kind” and seven of every clean “kind” on the Ark. What a “kind” is is nowhere specified, but it seems to be more closely associated with the idea of genus than of species, and sometimes even larger taxa than that.
Just grab all the beetles, and you’ll at least have a majority of species.
Actually Noah took 7 pairs of every clean kind. Think of how mean it would have been to have been the odd man (or woman) out if it were 7.
Even with two of every species, wouldn’t that equate to all animals inbreeding to reproduce? Does that mean that the original pairs of animals on the arc were far superior than decendents of those animals alive today since all descendents are inherently the product of inbreeding?
Not to mention Noah and his family! Did he have to do it with his daughters and stuff in order to keep the species alive?
That was Lot.
You are probably mixing him up with Lot, who was out in the desert with only his randy daughters around.
Noah’s family consisted of him, his wife, three sons, and their wives, IIRC.
I have to be second at saying all of them.
Thealogicaly - God sent one pair of each, and he won’t make a mistake.
Logicaly - Any animal that lives would have fit on the ark, and you didn’t say all at the same time. You have to watch those wily teachers and their logic problems.