How many, I do not know, but when the Dolphins had a decent product, they WOULD sell out.
As far as who they sell "season tickets" to...obvious that you have not come down to Miami in a while.
I've never been there. I only "know" what I read
From a Palm Beach newspaper, December 31, 2011
"Empty seats have become all-too common at Sun Life Stadium. The Dolphins were forced to buy thousands of their own tickets to avoid a local TV blackout for five of eight home games. According to The Sports Business Journal, the Dolphins saw a 14 percent decrease in actual attendance from last year to this year, the second-largest drop in the NFL.