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Whenever I read on how many to keep together it's always so the boy doesn't wear the girls out. So does that mean I can keep a lot of guys together without problems?

Lot's of people do just that. Normally you shouldn't have any problems. Denny

That's actually how they raise the fancy guppies on the Asian farms. The fish grow faster and when they select specimens to breed, they can be sure which male breeds with which female. Livebearer females can store sperm and have more than one batch of babies from a single mating, so this is the only way to be sure who the father is.

It's not wise with all livebearers, though. Male swordtails are quite competitive and fare poorly when crowded. Many hobbyists discover that they lose all their male swordtails until only one is left. Juvenile males swordtails, especially some of the rarer species, may delay development of their swords when kept in the aquarium with other males.

Young platy fry, kept in crowded conditions are more likely to develop into males and mature at a smaller size. This is Mother Nature's way of controlling overpopulation. Fewer females and smaller, less-competitive males, means fewer new fry to continue the crowding.