If we assume there will be 500 applications and that YC will be accepting 25 of them then that's a 1 in 20 chance.
Throw in the fact some applications, maybe as many as half, are broken (thirteen year old founders, clones of existing ideas, etc) and you've got a great shot.
Anyone that still has cold feet, get in there and apply!
On a serious note, can one apply as a single hacker who doesn't care about which project he takes on?