If you really want to increase your chances of having lots of Molly Fry, you may want to add, according to the size of your tank, two more females. Not all Mollies share the chemistry to mate, so adding a couple more females will increase the likelihood of it happening.. just add two more females and watch the magic happen.. he'll be dancing the Flamenco with a rose between his teeth for at least one of the three, possibly two.. well maybe not the rose between the teeth, but there's a dance they do where they bend their bodies around each other, once you see that, the mating ritual has begun.
Like Richard said if you put atleast 2 females to one male ratio you have a better chance. If they are like guppies or platy, they will make plenty of babies and then your question will be what do I do with all my babies. I would increase the females in your tank and let nature take it's course.