Just a few. I think it motivates you better when it's a small amount. If I play a map and I see it has in excess of 8 or more secrets, I usually don't bother. And I think it's good practice to make secrets worthwhile with something that can give an advantage in the level - make it rewarding. Unless, of course, you just wanna be funny and put a single health potion in there, in which case does make me laugh when I see stuff like that. :P