> But can I still limit Classic server to use 6 of 8 available cores if

I want

AFAIK, the CpuAffinityMask parameter is ignored by Classic server. It
is still possible to set process affinity in Windows Task Manager
(right-click), but I'm not sure whether the setting persists after
restart and how it influences other Firebird instances (as Daniel
wrote, Classic runs one process for each connection). I expect there
must be some utilities or register settings, try to google it out.

Regards,
Michal