I wonder how many ARM cores are built into a typical PC motherboard and other components these days.

A while ago, there was an announcement by, I think, Western Digital, that they were going to replace the ARMs in their HDDs with RISC V.

WD is a very active contributor to RISC-V, some of their "SweRV" RISC-V cores are open source: https://github.com/westerndigitalcorporation/swerv_eh1