Something I never really did was keeping F18A user statistics. What I can tell you is that there are about 504 F18A boards out in the wild. I have done 5 runs of boards based on demand:
1st 150 boards, July 2012 (I did the assembly and reflow myself)
2nd 100 boards, March 2014
3rd 100 boards, Sept 2015
4th 100 boards, Aug 2016
5th 100 boards, Feb 2017 (current batch)
It is a steady average of about 100 per year. Probably 50% are in 99/4A consoles, with the other 50% being split between the CV, MSX, and other. There has been some CV and ADAM increases recently, so it is hard to know where they are all going. Again, this is just speculation right now, so take the percentages with a grain of salt. To my knowledge, only one person has used the F18A in a system with a microcontroller (ARM CPU), and a few people here have used them in home-brew systems with CPUs like the 99105 and 9995. Everyone else is putting them in retro consoles and computers.
It is hard to know exactly what percentage of 99/4A users have an F18A, but if you want to know do a poll. Or look back in the forums, I think the question has been asked via a poll before (Omega?). Even though I created the F18A, I'm probably the worst advocate for it. I feel sheepish if I try to push it or bring it up constantly in forums. Anyway, other people here usually inject F18A advocacy where necessary and I greatly appreciate that.
IMO, do the project for yourself using whatever hardware and software you want and have fun. If other people can then use it or are interested, consider that icing on the cake. Don't make design goals based on what you think will serve the largest group of people in a given community. If you do, you will always end up having to target the lowest common denominator (which in the TI community is an expanded console). It is frustrating to me when I see new software developments with a stated goal of supporting everyone (which means unexpanded console), which then makes me wonder why we bother doing all this new hardware (mega carts, RAM expansion, storage devices, video enhancements, etc)? If no one will write new software to take advantage of it (usually with the reason that "not everyone has hardware xyx"), then what's the use of making new hardware?