According to Ironforge.pro, Mirage Raceway has the highest Horde population out of the English language Horde pve realms. But look how it compares to the pvp realms:
Mirage Raceway: 2326
Golemagg: 6894
Firemaw: 12347
Gehennas: 20488
That’s only a third of the next highest pvp realm population, and an eighth of the population on Gehennas. Maybe things will pick up a bit during the prepatch, but is it too much of a risk to commit to a pve realm at this point?
Regarding MR, I would think that 2000+ people are plenty. Horde side feels sufficiently populated, just now I was at the Xroads on my lock and there was quite a bit of activity. That been said, I’m very worried about the impending PvE to PvP transfers leading to a lot of players migrating away from the already Alliance skewed PvE realms, even though I am seeing a number of guilds Horde-side actively recruiting for WotLK.
TLDR: It seems safe to commit on MR, though I don’t know how PvE to PvP will affect the server.