Platy breeding question

Howdy,

Suppose you bought 2 adult platys and put them in a good environment.  Am I
guaranteed babies or do I need to buy a male?

background-
the tank is a  heavily planted 37 g. with little other animal life
(otocinculus, snails, those Japanese algae eating shrimp).  I am away from
town or busy or whatever, so my tanks have traditionally been all plants and
very little fauna.  I had golfish but got rid of them recently.  The tank
was in good shape but too boring after that, so I got these 2 platys to give
it some animation.

I remember hearing certain species' females store sperm, so that they can
have another brood even if they do not appear pregnant now.

I also intended to buy a male and female, but they both look like girls to
me now.

regards,
Keith Loyd