Does anyone know if this was changed during a previous update?
I recall the update during Halloween came with a crazy boom in NPC Population Density. It was amazing and the streets felt alive. Always something to see and encounter. Some might say it was hectic.
Just came back to the game after a few months and noticed barely any Inzoi on the streets. Would be lucky to bump into 4 Iznoi walking down the block. Seems dead 
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There’s a setting for population in the town sliders, make sure you put it to maximum.
Also Dowon is quite deserted on the NPC families side (the ones that live on the lots) as most are single young adult and not families. Bliss Bay and Cahaya are both better in that regards.
So set population to maximum and make sure you put large families in the lots you arent playing to make even more population.
But it is quite possible they nerfed it a bit, as some of the potato PCs could not cope as well as high-end users.
Also having this many zoi that close caused quite a bit of conversation lag as well.
The surge of zois was caused by the Halloween event with giving candies and stuff, otherwise it felt as usual.
I was not aware the entire town was more populated and did not see that in Bliss Bay where I was playing at the time, but I did not trigger the event.
Welcome back
I was researching for a potential change and saw this older post just passed the holidays when the original update was released.
too many npcs - Wishlist - inZOI Forums
Looking at the photos and comparing to gameplay for me now… definitely far less Inzoi around my game
Best I can suggestion is going to the wishlist forum and ask that for the population bar, setting it to maximum adds way more zois. It would be a nice use for that mechanic which is already in the game to begin with, should be an easy adjustment for them.
Other than that, adding more residential lots in maps (especially Dowon)
Or setting max family size to higher than 8 as a menu option would also make more population.
You can also set the romance option bar in town settings to the maximum, dev themselves said it increse changes to make babies from pre-generated families… unfortunately if you do that, you’ll be max romance in like 5 talks, which … well… sucks. 
My game might be bugged.
There are no Inzoi on the streets at all haha. I’ll play around more tomorrow and post as a bug if not resolved.
Thank you for your responses 
For example, I never see ghosts, neither in old saves nor in new ones, while I read that for some people they are infested with them. The only ones I’ve seen are at the cemetery and then a dead Zoi that stayed in the family home for a while.