When you determine the number of males & females, if you can keep 2 females for each male. You might have to exchange some depending on how big your tank is, but the males won't pick on the females so much. Platys do breed VERY readily, so if you don't want pregnant ones, stick with just males.
female analfins tend to be fan shaped whereas the males are more pointed. The male also has a visible reproductive organ called a gonopodium which sticks out of the anal area. Females will tend to be larger and bulkier than males and the males are more brightly colored.