How many people do you think can write / modify post-processors?

Used to could. I wrote long hand coding before I was ever introduced to Mastercam. But of course I rode a dinosaur to work and the wheel was a new concept at the time. So I think now I would struggle for awhile at it.

I "dabbled" in this a little with FeatureCAM driving HAAS controllers.

I had to self-teach. I was able to add the functionality we needed. I am usually pretty able at diving into unknowns like this. However, I would not describe my experience as intuitive, quick, or confident. My guess is that you are correct in your small percentage assessment.

Daen

early in 2000, I spent some time (almost a yr) to learn CAM software (Gibbscam software) and have a good understanding of CNC programming (edit and mofify the code) but never used it in production

This is why being a technical guy is exhausting. You spend years picking up a skill like this, and a few years later it's completely obsolete.

On a serious note, I think it is going away. People's ability to process things and wanting to learn more about what they do is changing drastically.

I have seen it firsthand in the past ten years this trend towards "there has got to be an easier way to do this" has crossed all boundaries and is showing up in every area I deal with. Whether it is a designer who wants an easier way to model, a shop foreman who says file paths to projects are to complicated to follow, right to the guys programming parts, there is this aura of there should be an easy button for everything  I do.

I first worked as a welder/ fabricator in a job shop years ago, my mentor there gave me 1 piece of advice that has stuck with me throughout my entire life,

" No one can ever take away what you know about your job." 

It sticks with me to this day.

If I understand the OP, he is asking about the post processor section - the translator that sits between the Gibbs/Master/Solid/Surface/Feature CAM software and the machine/controller specific G-code. More at programming the program that writes the code instead of writing the code itself.

I use to write/edit posts back when I was programming CNC machines.  Did it in SmartCAM and MasterCAM.  When we would get a new CNC, I would always have to tweak the posts.  Back then the posts were written so as to be easily edited by the end user, using notepad or wordpad...or any other basic text editor.  And CAM software came with a huge collection of post for every type of machine/control.  From what I've seen, CAM software is intentionally changing to posts that are not easily edited.  They no longer open in a simple text editor.  Some CAM software does everything they can to make it impossible for the end user to edit a post.  This provides the software company with additional revenue, since they charge the user for the posts and for customizing the posts.  CAM software no longer comes with a huge collection of posts for machines, you have to purchase each post.