I will say lucked out in finding earliest release. It will probably be impossible finding earliest releases of several other IT games other than earliest release versions already supported so far.
>SHVB dumped X-Men: Children of the Atom (950331 BRAZIL)
Impressive. Actual photo of a CPS-2 Brazil region pcb.
I waded through the file src/mame/drivers/cps2.cpp and will now say the future romset name in MAME will be: xmcotab It's a no brainer when seeing the romset names for other sets already supported.
So the next guess is how many regional set versions still yet to be found and added to MAME.
Going with this list and assuming there are not many multiple bug fixes involved pertaining to a language regional version....
- dumped X-Men: Children of the Atom (950331 BRAZIL)
Assuming Capcom's official idea was to have two releases of bug fixes instead of all the multiple unintended bug fixes, this means there three language regions yet to be found and supported in MAME in order for X-Men: Children of the Atom to be considered fully documented and complete.
(USA 950331) (Japan 950331) (Asia 950331)
Multiply that with number of other CPS-2 games and regional versions and that likely leaves remaining CPS-2 sets yet to be found and supported at about 75 to 100 imo.