Blunt: I seriously think that you need to do some more research before you get started. Thats an impossibly small space for a 20 plant grow, and to give you some perspective on this question my cabinet is 3 feet wide by 12" deep and I struggle to contain 2 or 3 plants in there!
Just trying to help you

There are some great threads here to read and will help you to get a better idea of what you would need.