Assuming you are level 20, and have access to any magic item, how high can you get AC (permanently or for a set duration)?
Magic items are those on the tables in the D&D 5th Edition DMG.
Spells, skills, feats and abilities available to a player character are eligible for use/consideration.
No buffing, no other help from an ally. This AC is achieved by a single player character.
Extra clarification: Time and magic items are limited to practical use. Think of it as a game plan for a campaign with a very generous DM.