[Inigo Montoya]

Special thanks to Taqwus as well as Zen's Item Database.

The max default MR is 20.

If that unit were made a prophet, then it gets a bonus of +5 MR in a province with a dominion of 10.

If the Magic Drain in the province is -2 or -3, then the unit would get +1 MR.

Resist Magic provides +4 MR.

Mage Bane gives +5 MR.

Lead Shield adds +4 MR.

Amon Hotep contributes +5 MR.

Armor of Souls increases MR by +5.

Amulet of Antimagic offers +4 MR (is this cumulative with Resist Magic?)

The maximum MR (before considering Iron Will) is 53. This assumes HHEBFMM slots.