I was watching a documentary about the SAS and realized I don't understand something about unit designations.

The SAS has three regiments:

21st (Artists) Special Air Service Regiment (Reserve)

22nd Special Air Service Regiment, part of United Kingdom Special Forces

23rd Special Air Service Regiment (Reserve)

Then there are other units, for example is the 24th Regiment of Foot.

How are the numbers assigned? Were there 23 earlier Regiments of Foot?