As of now we are aiming for 1000 unit limit on average. It will vary a bit between races - Orcs might have 1,200 limit, while Elves something like 800. Nothing is set in stone but this should give you a fair estimates.
And no worries - you won`t always have to fight this epic battle. You can always leave most of your units home, and just send 100 troops to attack other players or do quests - we`ll automatically find gameplay content (player`s to attack or quests to do) depending on how many units you are willing to send out.
Also, early mmorts game, as in good deal of single-player campaign, skirmish, you`ll be fighting with 100-200 units only.