i would say 5. however if you want another type of fish as well then obviously get less platies.
you can use this to work out your stocking:
AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor
i put in 5 platies to a 10 gallon and it suggested your tank would be 90% stocked.
Of course there are lots of other types of fish you can get - i think they different colours of platy are cool though.
also, here is something i found online - to give you some other ideas!
Stocking a 10 gallon tank:
Stocking 1
1 dwarf gourami
6 neons
12 ghost shrimp
1 snail- they are great algea eaters, dont bother your fish, and contribute little bioload to a tank. Better for smaller tanks than algea eating fish IMO.
Stocking 2
1 honey gourami- similar to the dwarf gourami, but stays 1/2 inch-1 inch smaller.
6 harlequin rasboras- small, good looking, and great schoolers. Another great community fish.
6 ghost shrimp
1 snail
Stocking 3
8 neons
6 ghost shrimp
1 snail
Stocking 4
1 dwarf gourami
3 platies- all male or all female unless fry are wanted
4 pygmy corydoras- stay very small, getting to around ¾ of an inch to an inch long. great for small tanks, but needs to be kept in groups of 4 or more.
1 snail