Rebuy/Add-on

I avoided this type of tournament, but i have a question. The rebuy part i understand, you lose your chips, you buy new ones up to the starting stack, ok, and let's say that lasts the first hour unlimited, all right.

Now, what about ADD-ONS? They start after one hour but what do they mean? Do you get unlimited add-on's for grabs? Can you purchase 10 times the starting chips?

Say after one hour i have 500 chips (starting chips 1500) while the chip leader has 85 000, can i buy 20 000 chips, or am i restricted to only one 1 chip count, 2, five?