guppies are ~1.5 inches of fish so (safely speaking):
in a 2 gallon, you can keep 1
in a 5 gallon, you can keep 3
in a 10 gallon, you can keep 7
in a 15 gallon, you can keep 10
in a 20 gallon, you can keep 14
in a 30 gallon, you can keep 20
and so on...
the more males you have, the less likely they are to damage each other's fins. and the more guppies you have, the less likely any significant number of fry are going to reach adulthood if just left in the tank.
if you really just love guppies, but have no major fishkeeping experience, i'd go for the 20 or 30 with all male guppies. it be a very lively tank with a wide variety of colors and you wouldn't be tempted into the fry business. plus, if you ever tire of guppies, you'll have enough tank space to keep many other interesting species.
if you do decide you want to breed guppies, be warned that most LFS either won't take them or will only exchange for other inexpensive fish. i bought a male/female pair and am on the 4th generation. typically i just clear out all of my males of a certain size and trade them off at about a 5 to 1 ratio. my females would probably just get thrown into the feeders; they aren't very colorful. unless you feel you just *have* to breed guppies, i wouldn't. its a bit heartwrenching, to tell the truth.