I hope for NPC mages... and player mages are to be immensly powerfull. Destruction magic has sort of faded away, because it is easier to summon and control Daedra, or bind humans to your will than to send a massive fireball once you reach a certain degree of proficiency. Magic has advanced as well, so I don't see 1 mage controlling 25+ dremora and hundreds of undead too far-fetched.
Mages by definition should be outnumbered, but each one is their own "evil mastermind" to a degree. Rebels are almost entirely flesh-and-blood, with a handful of machines.
Not every mage will be that powerfull - of course the leaders will be, packed with about everything magecraft as to offer to improve their abilities, but magic is still limited by it's human (or elven) componenet. Like sports, even with improved training and techniques that component has limits. There are ways around it (mantling, tapping some external power source, that sort of things) but that's for a select few.
What can probably easier to achieve is to have several normal mages synchronizing their castings to combine their spells (an exampe of the like is mentioned in one of the in-game books, with a rebelling army using each and every of it's spellcaster to make itself magic-proof. Including healing.). An alternative might be tossing raw magica or a special spell to one one them who uses that power to cast the heavy hitting stuff.
It doesn't exclude having some really mean and powerwful ones around, like the Ten from the
Black Company books (okay, maybe not that powerfull, as hard to kill remains possible)