Oh that's a tough one.
Honestly, given mIRC there are probably more people that benefit from having Windows XP support than those that would benefit from having ARM support (after all, ARM64 devices can run x86 code, even if sub optimally).
I'd probably lean on a poll or system check from folks to get an idea of the number.
However, I definitely still recommend getting an x86-64 version of mIRC up and running if possible! As there are enough security benefits under the hood in Windows these days to make that worthwhile.
To be fair, though, maintaining Windows XP support now would be similar to maintaining Kenbak-1 support when mIRC first came out.
