This is a chart of my count (up to the last post: Jun 17, 2021) being careful of the many duplicates, nuances and honoring when users repost to say they’ve moved to different distro(s).
Tab-deliminated:
# OS
32 Manjaro
27 Ubuntu
22 Debian
22 Pop!_OS
21 Arch
21 Fedora
17 Kubuntu
14 Linux Mint
11 MX Linux
10 EndeavourOS
10 OpenSUSE
8 ArcoLinux
8 Xubuntu
5 CentOS
5 Raspbian
4 Elementary OS
4 Peppermint OS
4 SparkyLinux
4 Ubuntu Server
4 Zorin OS
3 Solus
2 Fedora Silverblue
2 KDE Neon
1 Anarchy
1 Antergos
1 antiX
1 ArchLabs
1 Artix Linux
1 Chrome OS
1 Kali Linux
1 Linux Lite
1 Lubuntu
1 Mageia
1 NixOS
1 Parabola GNU/Linux-libre
1 Regolith
1 SolydXK
1 Void Linux