yeah, it really doesn't matter how many you start off with. . . . eventually the tank will have more guppies than water, especially if it is well-planted with a lot of hiding places for fry. if it was up to me, i would start out with 3 males and 7-8 females. that way there is some variety in the bloodline and everyone won't be inbred right away. it gives room to add more adults, without there being so many adults that the fry get eaten right away.
my experience with guppies - 30g guppy tank, also short-term grow out for a black ghost knife. he ate all the fry, and all the males (slow and easy to catch). we moved the bgk into another tank in mid-september. since then, the 8 females that were left have produced maybe 300 fry. . . . probably about 175-200 of them are swimming around in the tank in various stages of growth. kinda ridiculous, but female guppies can have several spawns from mating just once. it's mid-november, no males in the tank since maybe mid-august, and 4 out of 8 have babies on the way again.