True story: I did get up to 11 distros recently in a multiboot setup on my Thinkpad. I used Refind for the boot manager and everything worked well.
I had:
1 - Fedora
2 - Alma Linux
3 - Anti-X
4 - Slackware
5 - ElementaryOS
6 - Linux Mint
7 - Mageia
8 - EndeavourOS
9 - PopOS
10 - Lubuntu
11 - openSUSE
I’ll probably make another run at it and try to get up to 20. I need to lay out the partitions better. I definitely need to add Void and Alpine to the list.
I’ve never actually tried it, but I think you could use BTRFS subvolumes to multiboot without partitioning the physical space.
And then maybe even use deduplication across subvolumes?
That was the exact thing I was going to suggest. It does work!