Looks like it's 1,296 possible codes. As others have noted, numbers are used. JetBlue is 6B or B6. Some others include the late Canada 3000 (2T) and Roots Air (6J).

There's also some recycling. Example: The old British Caledonia code (BR) is being used by Eva Air.

There is also sharing by regional airlines. Example: WS is used by 3 minor airlines in addition to West Jet.