Well, ST has always had inconsistencies in ranks. Why should ENT be an exception?
It's too bad Sato and Mayweather didn't get promotions to lt., j.g., to show those "half-ranks." BTW, I had forgotten Mirror T'Pol was a lt. cmdr., so there's one half rank shown. So it's reasonable to assume that if that rank appears in the Mirror Universe, it most likely exists in the 22nd century Starfleet, as well.
As for the topic of how many ships there are in the pre-NX Starfleet, I would say there must be more than just a handful. And like in today's navies, I'm sure there are full commanders and lieutenant commanders assigned as captains/commanding officers of Starfleet ships, rather than all ships requiring full captains (once known as post captains in the old British Royal Navy).