I want to say you unlock a few slots every time you get a new job/class for a total of..45 at the moment.

I recommend making 1 set for each job you want to play and use the rest for glamour (so a swimwear, casual wear, whatever)

It looks like a lot of sets, but it's not really a lot of "free" sets (with 29 different jobs, 1 for each takes a big chunk!)