I'd expect no more than a fraction of a percent of Arch users compile the kernel and userland from scratch.
Gentoo may not have the highest percentage of users who compile from source because there's binary packages available now. Maybe Exherbo, Source Mage, or Lunar may have the highest percentage outside of LFS.
What Linux distribution^1 has the highest percentage of users who compile from source
Is it Gentoo
1. Besides Linux from Scratch