Actual Maximum Sim Count in a game?

Question 2: I know the game culls townies if you exceed your specified limit (mine's usually 200). It's likely that the same thing happens if you exceed your unofficial limit when you set it to unlimited. 

I can't answer question 1, but it's possible that there isn't one limit that applies to all saves for all players. Other factors affecting the save might cause the unofficial limit to be higher or lower.

Interesting question.

There is an hidden actual sim limit that counts all sims. It increases after each DLC release, so last I heard of it during Cats and Dogs time, it was somewhere over 800 Now it's definitely over 2,000...

The sim limit you see on Manage Households only counts sims you have in My Households section. These sims will not be culled if the actual limit is full. The game culls sims in Other Households section, housed or not. In addition, if a friend is about to be culled, they'll call your sim and tell them they're going off-the-grid and will be thinking about your sim as they're foraging through the forest. More about culling here.

Unlimited should really be unlimited. But the way you are telling that new townies generated immediately get culled the next day is... interesting... 385 should be far from the limit (and even I at about 1,000 sims have Other Households)... How many households do you have in Other Households section?

My sim count is about 750, it probably includes horses and pets. I never had a played house culled, but I would not know if NPCs are no longer around. I have enough not-played own sims to cover up for them.

I'm pretty sure that device specs will have a word in this. My computer is low-end but I think it helps that my driver has 2-300 GB free space. Best advice, back up regularly :)

And I