Well, we can't make calculation with only the number of people on each OS.
Some exemples to illustrate:
1) people using Linux are maybe less than others, but you have big animation studios in California who are ready to pay quite a lot of money to stay on Linux (because of the reliability, security, ...).
2) Working on IOS means giving between 30% and 35% of your income to Apple + it's a pretty closed platform so everything becomes complicated like generating our classical .tvp files that may not be accepted by the system.
3) We need a good support from the Tablet makers (hardware + drivers) , which is complicated on MacOS and Android f.i (but we have found some solutions, ok).
4) Some recent platforms are not ready yet to support all our technology (no CTG layer on Android yet or too slow to be used in an user-friendly way).
5) Remember than supporting an OS is also supporting it on all countries. Windows dev is complicated when you have to deal with asian langages and their OS/keyboard specificities.
Regarding the specs, we don't need any 3D card, TVPaint is only (multi) CPU based.
So you need :
-good CPU (2K video > 2 cores) (4K videos > 4 or more core)
-some memory (8 Gb or more is good, but less can work too)
-a fast HD or better a SSD.
it works fine on my laptop which is already 3 years old.