The exact size of the imperial army active in Othea currently is fairly large, but not extreme. We'd guess between 10,000 and 50,000 at least.

Evoker count on the imperial side isn't all that large. even going with say, 0.1% of the population capable of bonding to relics (which is probably a generous number), the count in the empire would probably be maybe a few thousand. And not all of those will have bonded, or have abilities of any reasonable use to the imperial army. We'd guess that there are less than 50 evokers in the invasion force, with most of them having only limited ability or usefulness towards combat. Most hardly have any ability at all. 
Currently, ten evokers powers have been determined what are in the Empire. The numbers previously here are what we'd imagine would make sense for the entire army, not what we're going to start out having. Around ten evokers is sufficient for this stage. They're rare, after all.

We don't want to work out the size of the Othean forces, but we also imagine that they'll have fewer evokers, considering how the Idris view them. Or at least, even if they have some, they're not fully integrated into the Othean army and thus can't be used quite as effectively.

On that mask: How quickly can he(?) move? And how will he compare to our character? Considering that she sees better in shadow than in light, we'd imagine her power might possibly even have been crafted especially to counter such as the one you've got on yours. She'll still not yet have any idea about what to do if she sees yours, though.