Just to complicate this further: Some trips have more than one record locator.

E.g) AUS to LHR via DFW. Suppose the first flight is AA with the connection being a codeshare marketed by AA but operated by BA. This will generate 2 record locators but only one e ticket number. First locator is for AA's booking system + website and contains details for both flights. Second locator is for BA's booking system + website and will show the BA flight.

AA (Sabre?) don't use numbers in their locators whereas BA typically do.

Several airlines and travel agents may share the same GDS/database (eg sabre) and will not have overlapping references.

Also note that many trips get cancelled before travel meaning the number of used locators may be further increased.

I don't think e-ticket numbers are ever re-used though.