O’Brien is a “chief,” Jonas and Cutler hold the rank of “Crewman,” and I think those are the only enlisted personnel who have lines and appear in more than one episode. Starfleet seems ridiculously officer heavy.
Is the enlisted/officer distinction different in Starfleet from traditional armed forces? And where are all the warrant officers?
The Enterprise-D could probably accommodate about 3-5,000 people if pressed to. It had a ton of space for cargo, evacuations, ferrying and/or hosting diplomatic conferences… TNG often makes passing references to these missions, but they often happen off-screen.