When the cycle is complete, there will be zero ammonia and nitrites, and some nitrates (probably will be high). There has to be a good source of ammonia, though, to start the cycle. That source can be a piece of rotting raw shrimp or fish or it can be liquid ammonia, whatever you choose. There are several stickies on this site explaining the nitrogen cycle if you're not familiar with it.
For a 20g, you could have 3-4 small fish. Check out the nano section of SW
on liveaquaria. com.