it's stated their number is 24, like a fixed quantity (talking about pryna, i think the fact she jumped from insomnia to tenebrae in brotherhood can be explained by her being a supernatural dog).
edit:
http://gematsu.com/2017/02/final-fa...s-vr-content-potential-pc-port-weekly-famitsu
In the movie there were areas that you couldn’t go to in the game. Will the amount of areas increase?
Tabata: “Those videos made use of glitches. There is a large map being used on the train route in the second half of the game. Since that area isn’t completely made, opening it up as an area you can freely traverse is difficult as is.
“We might remake playable areas in future expansions, but since that would take a year, it is not currently in our plans. For argument’s sake, if we were to prepare new fields, Insomnia would also be a candidate, but that also would take a year’s worth of work.”
...
https://www.finalfantasy.net/xv/fin...sses-potential-story-content-clarifies-games/
Audience: I would really like to know more about Insomnia. Like, at the beginning of the game it’s established they have an own currency, since the guys don’t know what gil are? I’d just really like to be able to stroll through the city and see the culture and everything…
FFXV team: (laughs) Ok, so an Insomnia-DLC? *Everybody laughs and cheers* Or a game like Civilization maybe? I’d like to play that, too! (laughs) Well, actually a lot of people have already been asking questions about Insomnia, so maybe we really should do something about that. We just don’t know how – like, should it be part of a DLC? Of a character-episode?
Audience: Hell yeah Insomnia Shopping Simulator!!
FFXV team: (laughs) No, but really, if you guys have any ideas as to how to implement that, go on and let me know!
old interviews but this gives me hope lol i honestly don't know how they should implement it tho.