They'll probably do a leisurely pace with further classes, and they will likely also come with expansions. FFXI had 6 basic jobs and 6 advanced jobs originally, which has since increased to 22 different available jobs. Four jobs were added with the first expansion (one was added outside of the expansion, actually, so people could play it without buying the expansion), three with the third expansion, and two each with the fourth and fifth expansions.
It's quite feasible that ARR will follow this same pattern. We'll probably see a new class/job with each expansion, and they very well may add in more jobs to branch off different classes, too.
Edited, Jun 25th 2013 10:40am by Catwho