maximum magic resistance?

[Amhazair]

I don't think there is a hardcoded maximum. (Although you might want to ask modders whether there is or not. I think most of the values below can be modded just fine, but don't quote me on it.)

The maximum you can achieve more or less 'normally' though would be: base 18* + lead shield (4) + starshine skullcap** (2) + armor of souls (5) + Antimagic Amulet (4) + any of the MR boosting spells (4) = 39 + being in a heavy drain dominion (2) = 41. On top of that pretenders get a bonus for being in their own dominion, but I can't remember offhand what the maximum is.

I think that covers all possible boosts. Of course, any setup like that would leave you quite weak to anything not involving MR.

* 18 is the 'normal' maximum base MR for the heavy hitters in the game. There are a couple of exeptions, (horrors for one) but they're really rare, and I'm not sure any of them have full slots.

EDIT: The Iron Angel (Ulm national summon) has base MR 20 and full slots.

** If you're cold blooded and want to craft an extremely expensive artifact you can wear the Jade Mask which gives you + 3 MR along with some other benefits. (Allthough, are there cold blooded units with base MR 18 who have full slots? Intresting quiz question... )