how much semen is enough?

Yes that's the average, but there are other factors too consider - length of time between ejaculations, stress, diet etc.

It also varies depending on how long the sexual activity has been taking place before you ejaculate. The longer the activity is engaged in, the more fluid builds up to be released.
Diet and hydration play key factors as well. If you don't drink enough water, it will definitely reduce your semen output.