I just tweeted Elon a suggestion that they reduce the number of versions of software they publish and support. He wants to reduce cost, this is an area he could significantly cut costs. I have managed large software development projects for the Navy and I kept the number of active versions we were supporting to no more than 3 and preferably just 2. When you have 6-10 versions it takes a lot of resources to just to track which versions have which bugs and as we see in the software releases, things that worked in one release are broken in the next. With that number of teams it is easy to have one fix broken by a different team fixing a different bug. The communications become a nightmare. They could narrow their focus, get bugs fixed more quickly and get new features rolled out faster. A win for everyone. They could put the extra resources it would free up to more thoroughly testing code for both bug fixes and whether they have introduced any new ones.
12.1.1 is mostly for S and X cars. That's rapidly being upgraded to 16.1.1, which is exclusively for older S and X cars.
12.1.2 is mostly for newer cars (AP 2.5). There's been a limited roll-out to 16.1, but it hasn't gone wide yet. That's a pretty standard strategy for Tesla, to see if there are problems before switching everyone over.
All the other versions appear to either be old orphans where the cars may have gotten "stuck" on an old version for one reason or another, or the occasional odd-ball new variant (e.g. 12.1.3) which goes to very few cars and has an unclear purpose.
So for the most part, they've gotten it back to two versions, which then undergo an upgrade cycle.
Frankly, I'm not sure why Tesla insists on having one firmware scheme for all models. Many things are very different between cars: batteries (think an old S vs. a new 3), autopilot sensors, MCU, autopilot computer, etc.. There aren't third-party apps that require compatibility, and the cars don't need to share files with each other. I wouldn't be surprised if at some point there are some permanent forks, perhaps associated with who has the FSD computer.