With no gear, CP or other bonuses crit damage is 50%. This means that if a non-crit hits for 100 damage then a crit of the same attack will hit for 150.

The cap is at 125% crit damage (above a standard non-crit), or a 100 damage hit going to 225. To hit this cap you must invest in bonuses that give 75% additional crit damage.

Edit: For example, a common strategy is groups is to use:
- Major Force (Warhorn & Saxlheel) 20%
- Minor Force (Barbed Trap) 10%
- Minor Brittle (Frost Staff Chill) 10%
- Elemental Catalyst 15%
- 1 Piece Medium Armor 2%
- Fighting Finesse CP 8%
- Harpooner's Wading Kilt 10%

Other sources could be 6 pieces medium armor, Backstabber CP, Khajiit race, Order's Wrath set, various class passives (Templar, Nightblade, Warden), Shadow mundus and Axe weapons. None of these are needed often, and the 125% cap has made this largely a list of useless things.

Eso-hub also has a really good guide that lists out all the sources (minus the orders wrath set it looks like). I’ve found that helpful just for figuring out which crit modifiers are easiest to use to reach the threshold for a given build/situation.

https://eso-hub.com/en/guides/critical-damage