I did all of Canada's Wonderland in one day, minus the kiddie cred, so 15?
I also have a few two park days, but they usually involved smaller parks. Of course now that I think about it I did Kennywood (6) + 3 hours at Cedar Point (5) for 11. I also did Knoebels (3), and Dorney Park (6) for 9 in a day.
I have mapped out a way to beat the Guinness World Record (in a 24 hour period), without the use of a helicopter (like the current record holders used). Depending on flights and or kiddie creds the trip would be anywhere from 77-83 coasters. I have debated using my tax return and giving it a go next year, the only thing stopping me is I already have about 75% of the creds on the trip.