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... 

)