IIRC the magic number is supposed to be 6. at least thats what it used to be. you could register up to 6 cars in your name before needing to be a dealer.

but that doesn't mean you have to register every car you have in your name. I've bought and sold over a 12 cars in a 6 month time period before and never titled any of them in my name so as far as the DMV is concerned, I have only owned 1 car over the last 2 years.