Guppies will reproduce like crazy if you have the water correct. I once had just guppies in my 10 way back when I first got it, and I started with 2 females and 1 male, and a few months later I had within the 20s worth of guppies... So if you plan on having them breed, make sure you either have another tank to put them in, or a LFS that will buy them back from you.
Another good option is to just get all males and put them in the tank because a) they wont breed and b) theyre a lot prettier than the females.
as far as stocking goes, assuming you get all males, and you dont get any other tank mates, you could easily do about 6 guppies. however, you will most likely need a few bottom feeders such as some cory cats, and in that case id say go with about 4 guppies and 2-3 cats, and make sure you keep the water chemistry perfect because from my experience guppies are messy little buggers.