I think Tribes only gets big populations on weekends. Their double XP weekends or whatever are the times when you don't end up playing the same people every day throughout the day. Otherwise weekdays I'll see the same 10-15 people very often, on weekends maybe 5 of that 10-15.
It's more consistent now that they group by rank, but it was still a pretty regular group of people playing at certain times before that change.
So I'd guess they maybe have a steady group of 5-8k players per server hub, with it spiking up to 15-20k on the XP weekends. Really hard to judge on how many people overall throughout the day, but I'm thinking 5k people seems pretty reasonable for a weekday.
And looks like I won't be getting into PS2 beta for at least another week as a vet.