Rule of thumb is 1 square foot per female and another 1/2 a foot for male. This means in a two foot you can only really have a pair. And if you breed in a smaller tank as Rod said you gotta get that female out quickly. Some fish will never breed in anything smaller than a two foot.

If you want a harem you are talking a 3-4 foot tank.

I would suggest always leaving the female in - they very rarely eat the eggs/young. Consider removing the male.

HTH
Steve