The best ratio for guppies is 3 females to 1 male.
Where do you plan to get these guppies from?
Petstore guppies are not the healthies of creatures and normally die once placed in your tank no matter how great your water. The longst I have ever had pet store guppies live is about 3 momths. Hopefully you can get atleast one female to give birth.
When buying guppie females from a petstore they are always pregnat...Who knows what will come out
Its like a free grab bag lol.
When looking at these fish look to buy larger females...it is normally a sign of healthyness and they normally carry larger amounts of babies.
When buying males buy a color pattern you like. But please do not be upset if the male dies. I have had males live longer than females. Look closely for diseases. ICH is very commong among guppies in petstores. When guppies get ich it can be difficult to cure. Guppies from the petstore that get ich normally die but it is possible to cure them!
Infact, I can't get a female fancy guppy from my petstore to live. Either of my petstores!
The best of luck to you.
Oh
BTW if you want a good strain of a guppies check
www.aquabid.com or look for a breeder within your area
I recently found out that about an hour away from me there is a guppy breeder club...Hopefully I can join and obtain some nice guppies