There is something in the Studio Neo architecture that causes that. The bug is elusive, sometimes depends on what other programs you are running at the same time, and sometimes even depends on whether you loaded the scene right after a reboot or if you ran some programs already. This is kinda a well known issue.
You can check what your absolute maximum character count is by disabling all the mods first. You can go into KKManager, Mods tab, disable all.
Now, let's see if we can reproduce your crash - can you save your scene just before the crash then post it here together with the character that causes the crash once added? Because despite what I said above about the SN2's bug there might be something about your particular scene that makes it crash.
I also have a few scenes like this, from various creators, that make the SN2 crash when trying to load the scene, despite the scene having only ~3 charas.