The letter/digit-space is per GDS, not per-airline/industry, however different airlines can have particular additional restrictions on codes. Some airlines don't allow 0 and O (i.e. Qantas), others don't allow I and 1, and others always/usually have the same ending letter (unless code-sharing).

Another thing to note is (unless something's changed in the 15 years since I left the airline industry) that non-trivial booking changes (i.e. not just passenger information changes) generally result in a new PNR being generated to replace the old one, effectively using up the old code.

Each record locator is unique not by airline but for gds, in this case Sabre. Each record locator reference a reservation which may include many flights and people that fligth together in a trip. And a reservation is done 1 year in advance. It’s not a easy math

Would it also include digits? If so, 36^6, which is 2.1B