Just be clear on one thing: regardless of whether you get more or fewer to start with, your tank will still be overpopulated within only a few months unless you have plans to remove the bulk of the fry. If the tank is completely barren...no plants or other cover...many fry will be eaten but some will always survive, grow and eventually add to the population problem. Guppies aren't particularly effective at predating on their own young.
So, if you want to breed them, the question isn't how many fish to get...it's how many tanks to get.