After 300 the game wont save anymore (even if it says game saved).
Even overwriting a different save doesn’t work (the number wont go past 300)
Are you using the save mod? I heard that caps at 300 due to a bug, however I haven’t heard about the game capping out at 300 natively yet.
Supposedly you can load your current save, then delete all your saves from C:\Users\<user name>\Saved Games\kingdomcome\saves\playline0 (Back them up!! I’d move them instead of deleting), and when you try to save a new game the counter will start at 1 again.