thats a pretty good stocking level already. a good rule of thumb for beginners is 1 inch of fish per 1 gallon. now keep in mind this is a generalization and many of us over stock (especially those with cichlid tanks) if you have very good filtration on the tank you could squeeze a few more guppies in. your platy's are very messy fish, both when eating and the amount of waste produced.
hth
richard