I've assumed that under normal conditions, only one transporter unit is active at any time. The others are shut down for maintenance and de-gaussing, et cetera. They can probably be fired up if emergency conditions warrant it, but generally just one. The crew who would need to know what transporter was active at the time would already know.
I think so, too, and as a crew member I guess I'd actually appreciate the transporter crew to do heavy maintenance on a device that will scatter my atoms through space.
However, if you believe there's only one engine room ("engineering section") the label "transporter section" could equally indicate just one, given the lack of a number (in contrast to the briefing room numbers).
But, indeed, the transporter room corridor differences (one with a red door turbo lift befind the A-frame, the other with just a blue door and no turbo lift sign) suggest there to be more than one.
According to The Making of Star Trek
the producers envisioned 11 transporter rooms (of different sizes) and TAS mentioned at least a "transporter room number four".
I'd like to believe four transporter rooms to be the minimum, but that's just my personal opinion.