As the game forum is pretty much dead, posting in general for visibility in hopes somebody encountered this as well.
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Once you create 200 saves, the game gives you a message that you reached the maximum number of saves and can't save anymore. Just delete the previous saves you might ask? The issues is, you can't, (the name of the save game just changes to "null" but the save is still there and can be loaded without issues) and the save slots stay there permanently. The game does not allow you to make more than 200 save files.

This makes it completely unplayable as I have just reached this limit. Not even deleting the saves from the save folder helps. The problem is probably in the "save.dat" file in the save folder which seems to be handling the logic of the saves (if you delete that, the game doesn't see any saves at all) but nothing can be done with that (without hex editing). As if the game can't properly modify and remove the save entry from this file when you click the delete save button in game. And if you hard remove the saves from the folder, the logic is still saved in the faulty save.dat file (slot containing nothing but still counts as a slot) so there are no easy solutions. Any ways to get around it? As of now, I can't continue playing the game because of that and it might as well be considered broken and unplayable (once you make your 200th save).

The entire save system in the game is pretty stupid anyway, constantly making new saves and including BMP screeshots (which are 5 MB in size each due to my resolution) without the ability to name them or manage them in any way.

EDIT:
Figured out a workaround.
1) Backup your last save
2) Uninstall the game. Install it again, start a new game and make a single save (the goal is to get a clean "save.dat" file that expects only one save to exist.).
3) Take your original save and replace the one you just created after the reinstall.
4) Now you have 199 more saves before you have to repeat this process again.

Annoying but at least I can play the game now. So if anybody encounters this issue, you can use this workaround. Will also post this into the game specific forum as I haven't seen anybody talk about it there.

Surprised not to see something like this mentioned more often if so. A quick search just turned up a Steam review mentioning it.
You mean you can't overwrite saves when you (manually) make them? That'd be one heck of an issue.

Yeah, I've also found one thread on Steam where someone asked about this but it was without response.

All you can do is just click the Save Game button in the menu. It creates a new slot every time and you can't even name it, so no overwriting. Once you reach 200, you have to do the workaround I posted if you want to keep saving.

As I am playing on a very high difficulty, I save a lot (reached 200 saves before even reaching the first big city).

There's another workaround: if you delete from the beginning of the save games, it will let you save over those spots.

It's still a pain - and very possible to accidentally delete important saves this way.