[margalabargala]

Alaska uses 6 letters. 26^6 is a bit over 300 million codes. Each year there are about 5 billion air passenger boardings, so while the whole of aviation runs through that space every 3 weeks, any individual airline takes much longer.

[pixelesque]

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.

[airline-it-guy]

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

[lewisleclerc]

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