Yes, the jobs are limited based on the number of schools. In Division III, there are 228 jobs - once those are full, no new coaches would be able to signup in that world. As coaches are successful and accept jobs in Division II, their old jobs would then be available to be filled by new coaches. The same would be true of a coach which decides not to play in a subsequent season (see "What happens if I don't renew my team?" below) - his job would then be open for a new coach.
If all of the jobs are taken in a World, you will see a note that your name has been automatically added to the wait list. As jobs open up as other coaches accept positions at other schools, the coaches on the wait list will be emailed regarding the job opening. Jobs are filled on a first come, first served basis.