Are the buses limited capacity? Do they sell out?

I have used them once a month for the last 6 months and have not seen them sell out. They are however, running every half hour and NOT every 15 minutes like they used to.

I have a flight at 5am what hours do I have to be on the fly away

How much cost park a car for a day?

Buses do have a capacity but they are constantly running every 20 to 30 minutes

Yes, there's seating capacity. During peak season, they do sell out.

Its possible. I would get there as early as possible to reserve your spot in line. But there is a bus every 20 minutes