Catholics have one denomination, protestants have millions. How come?

Depends how you want to look at it. The catholic religion has members that believe everything the catholic religion teaches, but it also has members that don't. There are catholics who don't believe in purgatory, catholics that think women should be priests, catholics that pray to saints and Mary for help and some that don't, etc., they just don't start their own denomination. Until you find out what each catholic member believes who knows how many denominations there would be.