How many creatures should be in a draft deck?
Usually, my draft decks tend to aim for 16-17, 6-7 other spells, and 17 lands.  Generally, if you end up with 19-20+ creatures your deck is going to be aggressive but insufficiently versatile.  If you end up with 11 or fewer creatures your deck may have lots of "answers" but a shortage of ways to actually win the game.  As ever, striking a balance is key.

There's nothing wrong with having 12 creatures or something if your deck is packed with sweet removal: take a red/black deck for instance; the creature quality in these colours tends to be low, but you make up for it in the strength of the removal cards.  Obviously it'd be daft to be adding bad creatures to a deck to "make up the numbers" when you have excellent removal at your disposal: being able to take out one of the opponent's best creatures is almost as good as playing a creature of your own.  

Also, make a note of cards in your pool which aren't creatures but basically count as creature cards.  Mind Control is one of the very best cards in the latest Core Set because not only does it remove an opponent's creature, it gives you one as well!  And most sets have an instant or sorcery that generates token creatures these days.  If you draft a lot on Magic Online where it keeps a running total of creatures for you, you'll be familiar with having to take that figure with a pinch of salt - because often it lies!

Anyway, 16-17 creatures or equivalent is definitely "average and typical".  If you're often drastically over- or undershooting that baseline, then (unless you've created some kind of insanely broken rogue decks that crush all comers 3-0) I'd definitely try to steer closer to the 40% creatures mark and see if that works out better for you.