There are limits to the thickness of coats. I don't have that info at hand. As Randall said, it depends on what was put down previously. If your current paint has imperfections such as small bubble marks, flaking, etc., you'll want to sand it off.
Epoxy primer is a good base over old paint. It's pretty nasty stuff too, if you don't have proper respiration equipment. When sanding the old, if you finish with a 400-600 grit, you should be ok.