There will, at a bare minimum, always be two "standard" Linux ones. One Debian-based and one RedHat-based. That said, a lot of the differences are just... derivatives of those two.
First RedHat - In the case of RedHat's relationship to Fedora and Centos Stream, those two are the upstream guinea pigs before things end up in RedHat Enterprise Linux. From there it ends up in Oracle Linux, Rocky, Alma, etc. The "RedHat" ecosystem is similar enough that if you learn one you've learned them all. The reason for the variants is the heart of it, RHEL, is not free (as in beer). But a bulk of the development and focus for this is done by paid RedHat engineers, even for the freely available Fedora and Centos Stream.
The other side of the coin is Debian based, which is the exact opposite of RHEL. Debian is the core, free in both senses of the word, and everything Debian-based is a subsidiary of it* (Ubuntu does enough of its own development that its kindof its own thing).
Because of the fully free nature of it, there will be tons of forks and branches of Debian.
Devuan for example came about because a group of people didn't like systemd and wanted to keep sysvinit.
Ubuntu came about to be a polished Desktop version of Debian, then branched into a Server variant, and now is largely parallel to Debian.
Mint came about because there still wasn't enough polish on Ubuntu or Debian for desktops, and thus there's versions based on both.
In theory, if RedHat went away, that entire side could die, and we'd be left with Debian. In practice RedHat will never die, thus there will always be two.