How many cryptids are believable?

I put this one on some FaceBook crypto forums. It'll be interesting to follow the comments:

Here is something cryptozoologists don't discuss much. The whole basis of our field is the idea that we shouldn't dismiss reports of unknown animals, including large animals, without cause. We know there are many unknown species, and it's a pretty sure thing there are some large ones, even large land animals. But we don't talk about the fact that it seems well beyond possibility that every continent still has five or ten large-to-very-large animals undiscovered. For example, worldwide I do think we have at least one large primate, with the chance of a couple more: I don't think it's remotely possible that we've missed two or three large primates on US territory alone. I do think we have at least one very large marine animal, but not a half dozen. There are some interesting reports of very large creatures in big lakes, but the idea of a big creature in almost every large lake, even if we narrow it to lakes with multiple independent reports, is absurd. Yet in our zeal not to miss anything (a zeal I have often expressed) we overlook this impossibility of the whole planet still swarming with big (50kg+) unknowns. Thoughts?

Update: Wow, did I draw a crowd of responses when I put this on Facebook.  Loren Coleman, Chad Arment, and other well-known cryptozoologists chimed in.  We discussed the validity of reports, the question of how big things can be missed, and so on, but I don't think any of us has worked out the answer to the core question yet.  See: http://www.facebook.com/groups/261237557287228/