How many CR 12 monsters are in the D&D 5e SRD?

2 CR 12 monsters appear in the SRD 5.2.1 — Archmage, Erinyes.

How much XP is a CR 12 monster worth?

A CR 12 monster is worth 8,000 XP per the SRD's Experience Points by Challenge Rating table.

D&D 5e SRD · Challenge Rating

CR 12 monsters

2 monsters at Challenge Rating 12 — worth 8,000 XP each (16,400 XP combined). Click into any of them to roll attacks, damage, and saves straight from the stat block.

Name	Type	Size	HP	AC
Archmage	Humanoid (Wizard)	Medium	170	17
Erinyes	Fiend (Devil)	Medium	178	18

Stat blocks from the System Reference Document 5.2.1 © Wizards of the Coast LLC, used under the Creative Commons Attribution 4.0 license.