2nd cycle and so on

i have read that it is possible to gain anywhere from 20-30 pounds on first cycle. what about after that, what are gains like on the 2nd 3rd and 4th etc. cycles?

Depends on what you're willing to do. eg

Adding more drugs, duration to your cycles.
Eating more food.
Using more powerful anabolic agents to your cycle.

If they are all bulkers, you can gain 20 more lbs but watch out as even from first cycle you could gain fat from eating big. If I had it to do all over again, I would bulk then cut and so on (cycle 1,3,5 bulk 2,4,6 cut)