Before everything that happened to it, yes. Most of the mages are now dead, killed by the scourge or various events. Hell, even Sunreavers where a big part of the population, and now they aren't living there anymore. It's nowhere near a big country, it's a city like any other, way smaller than any other capital.
I'm pretty sure that if they indeed had these many guys to send to war, we'd at least see one of those in ICC, which we didn't. A lot of them probably died in Ulduar, and back then, they probably weren't much of them left after all that happened to the city.
I'm pretty sure she's not the strongest mage in the world. She's good yeah, but many other mages in history were better than her. She also has almost no experience for war, while Sylvannas is a ranger AND a banshee Queen (empowered by strong unholy energies, like every major undead). She has a LOT of experience in battle, because she's old and has seen many wars. She even looked at death in the eyes several time, she's fearless and she knows what she's doing. Also, both managed to incapacitate and cope with the Lich King itself.
To me, they're pretty much even in terms of strength, except one of them can die a few time. Don't forget Jaina almost died because of some lame warlocks in Cycle of Hatred. She's totally mortal and can be overpowered like anyone else.
Sue her, at least she did something.
She wasn't undead yet. Now she's a banshee queen, not a mere ranger.strongest human mage in the world > hunter that couldnt even fight arthas when he was just a death knight.