This question is impossible to answer.
No-one can tell, 'cause no-one knows.
Besides one needs to distinguish FreeBSD, and its userland, it's the question:
How and even more who do you count?
Anybody at this very moment?
Doing what? Improve the code? Adding features? Patching bugs? Review code? Organize resources? Doing public relations?....
Or everybody who was ever involved, since...?
Also single programmers published a script for it maybe some use for others?
There are dozens, if not hundreds of those, doing sometimes really useful work.
To count, or not to count?
E.g. only LibreOffice, KDE, and X are running projects for decades, counting a
lot of people.
Of course their projects are used by many FreeBSD users, although they are not FreeBSD.
And yet we did not talk about how you count those who code on more than one project...
Predecessor projects, forks,...
It's a simple law of physics:
The closer you look, the harder it is to draw a distinguished line.
And I also don't understand the second question.
It can only be answered as vague as it was asked:
Depends on the size of the project - what do you want to do exactly? How much time anyone involved can work on it? How reliable are they? Is it payed for? (Of course not.) When shall be its release?...
Or, to put it in another perspective:
If you want to know the man-power you need for some OpenSource-project just do what all project-managers do: Evaluate similar projects.
Good start was to research
Linux distros.
Some of the FreeBSD-based-turn-key-attempts will also do.
Or - to be more specific - do some historical research on X, and Wayland, and compare those - the projects, not your personal judgement on their usability for your personal needs.
I could also say:
twelve
Which of course were obviously made up bogus.
Or to put it even more briefly:
I recommend you rather think about to add your energy into either X, or Wayland to help one of those to become better/survive, instead of thinking about to start a third variation, which will bring up more issues, weaken both even more, only, by a third alternative, which eventually may also end up as some not-consequently-brought-to-some-useful-result, instead.
Another law of nature is
you either get something comprehensive, capable of dealing with almost every expected aspect, thus making it large, and complex.
Or you can get something small and simple, which can only be achieved by sticking to the exactly delimited specification, consequently skipping everything else.
You cannot have both.
Most projects die by having an irreconcible mismatch between too optimistic estimated, and actual need of resources. They simply starve to death by naivly believing a not clearly specified target, which is also every now and then relocated again, could be reached, also within way too tight schedules, and way too optmistic reduced resources.
While the simple truth is: One cannot reach a target that's not there, ever.
The question I am way more interested in is:
How many users do FreeBSD have?
Especially on the subject of estimating if one project was worth it
this was the very first question I had asked.
Before I would even think about to think about to start such a project
I would first do some research on already existing ones.
Are there any other GUI attemps besides X and Wayland?
Figure out their status, situation, and circumstances.
Think about first to join, reactivate, continue one of those.
At least find out, if the idea what came to my mind to start such a thing wasn't already tried by someone else,
so to learn from others, not doing the same mistakes again.
