Mollies can stay pregnant their entire lives from a single mating. They often come from the pet store pregnant.
They can delay fertilization from a mating, and they may produce a single fry at a time, or a dozen or more.
If you're going to raise them up, I would suggest setting up a tank for them, at least a ten.
Keep them warmer (80 or so) and they will grow faster. Usually takes about three months to get them big enough to avoid adult mollies, but it you have bigger fish in your tank, you may need to wait longer.
Also, if you keep other kinds of small fish in your tank with the mollies, they will be less likely to pester the fry once you introduce them to the adults.
In my experience, once I had some half-inch fry in the tank with the adults, the adults don't pay much attention to any new smaller fry, even the ones born in their tank.