There's an excellent article here: https://www.linkedin.com/pulse/best-cnc-programmers-also-know-how-edit-post-colin-gilchrist

There are CNC operators, CNC programmers, CAM software vendors, CNC equipment retailers, machinists, CAD users, engineers etc... in the industry. I'm wondering how many, as a guess, can modify and how many can write a post-processor? It seems like it would be a very small percentage of these people in the manufacturing industry.