I've done both the unlimited and the daily use in the rainforest. Unless you are sure that you will go every day, the unlimited isn't a good deal (if you miss one day, the daily rate would be cheaper.)
I'm sure there is a capacity, but I've never seen it crowded. The only thing that fills sometimes are the heated chairs. Usually if you wait just a few minutes, one will open up....unless someone has fallen asleep in one of them. The steam rooms are never crowded. It is very enjoyable and a great way to relax.
Also.....if you have a "hands on" treatment in the spa, you can do the rainforest at a reduced rate on that day.
You don't need (and can't make) reservations for the rainforest.