you can nest jails quite a lot. Although i have not tried it (and cannot find proper sources on the max limit of the amount of jails you can inside a freebsd system).
If FreeBSD has a maximum limit on PIDs that includes those running in jails, you'd probably run into that well before you run out of memory (especially if the goal is to run "as many jails as possible", you can make a whole bunch that effectively do nothing at all).