OK, to be more correct, how many GNU/Linux distributions are there?
Historically, the OS elements were built first (GNU = GNU (is) Not Unix) by Richard Stallman. He was trying to develop his own kernel years before Linus Torvalds had worked on the Linux Kernel. Putting the two together we have GNU/Linux. Distributions then came out using different Desktop Environments, Gnome, KDE, ice-wm (window manager), Enlightenment, MATE, Budgie, Cosmic, xfce, LXDE, LXQt.
"The first desktop environment to be used with GNU/Linux was the K Desktop Environment (KDE) . KDE was developed as a response to the proprietary nature of the Qt toolkit used by the original GNOME project. GNOME was started on 15 August 1997, but KDE preceded it and was more widely adopted early on due to the licensing issues with Qt at the time. However, KDE's initial development began in 1996, making it the first widely used desktop environment for Linux distributions." [Source: Brave A.I. search engine]
KDE stands for 'Kool Desktop Environment'. It was more familiar to Windows users as it had one panel at the bottom of the desktop with a Win2k style menu and a familiar look-a-like system tray. Gnome had two panels, it's main one at the top showing Applications, Places, and System. The bottom panel tended to just have workspace switcher. Gnome led the way in terms of Accessibility features over KDE with Orca screenreader, but in the early days, the voice of the screenreader in OpenSuSE 9.3 Professional had a squeaky voice, not like today with many different voices and even English Dialects! Nowadays, apps such as Orca are DE Agnostic. You can easily install it if it is not present in Plasma (KDE). When you see distributions offering TDE (Trinity Desktop Environment) that is basically what KDE looked like in the early days and even has the same login and exit sound used by distributions that ran with KDE. OpenSuSE 9.3 Professional came with the option of installing KDE or Gnome. I made the mistake of formatting the drive with ReiserFS and lost a lot of data when it crashed. Ext4 File System is by far the best FS for GNU/Linux. Behind different distributions are different package managers that set it apart from other distributions further. OpenSuSE is a Red Hat form of GNU/Linux, using .rpm (redhat package manager) and Debian based distributions used .deb packaging. Others include PACMAN (Package Manager) and Arch uses something called AUR. So, it is not just the Desktop Environments, or the different distributions that make up the differences, there is a lot of stuff 'in the mix'. Not forgetting Gentoo, again with different DE's available. In the good old days, systemd did not exist, nor did Pulse Audio. Systems would boot with SystemVInit, a single boot process, not a goo ball that systemd is, taking over your machine and limiting what you can do with it. The earliest GNU/Linux sound software was called OSS (Open Sound System). Sound in the early days was not automatic, you had to tweak it in order for you to get any sound at all. It was great fun!
Then things improved with ALSA (Advanced Linux Sound Architecture) which if you are a musician allows multiple streams. Then game pulse audio, created by the same person who created systemd. Like systemd, pulse audio was pushed by Red Hat across the GNU/Linux spectrum, RH wanting to become the Microsoft of the GNU/Linux world, now ironically swallowed by IBM! And the guy who invented systemd and Pulse Audio now works for Microsoft!