I think 14 or so was my top. It was exhausting. I don't think it matters if you do one park, or multiple parks. You need some down time. Even if it's just taking some time in the A/C for a show or two around lunch or dinner so you can rest for a while. You will be there during one of the hotter times of year. That's enough to wipe out anyone. Even if you make it, you might not want to do anything the next day.