The dominant Arm core in the world is a Cortex-M (or Cortex-R) which are single-core. They are 99% of the time on a die with far less <512K SRAM, and run an RToS or baremetal.
These outnumber x86+Cortex-A by probably a factor of 1,000.
That's a good question. I guess we would need to compare all the arm multi-core in smartphones & some laptops, vs all the intel laptops/desktops/servers. Hmmm... tough one.
Virtually all modern (last five years, for the low end) smartphones have multicore chips. It's estimated that there are around _7 billion_ smartphones worldwide, with another 1.5bn sold every year. So I don't think it's a very tough one.
