How many NPC models?

[Arvaos]

Will it be hundreds or thousands? Will the city be bustling or will the streets be quiet similar to Deus Ex?

[[OG] Brandoid]

will probably depend on your graphic settings

[Defective Dopamine Pez Dispenser]

Exact number unknown, and how crowded streets are depends on what district you're in, and even the time of day. Pedestrian NPCs are not all unique, however. There will be repeating models and various combinations of clothing. This is not a game where every single NPC is unique and has a life or is fully interactive etc. Some are, but many pedestrians are basically just fodder as in other open city games.

Some good (older - newer builds look better visually, but it still communicates the general idea) footage of the game's crowd simulation. (From 4:38 on.)

https://www.youtube.com/watch?v=Fko23gfERE4

And at various moments in this one too (particularly around 28-29 seconds - this is a newer build, note the improved visuals.)

https://www.youtube.com/watch?v=jNC6MEA4rQk

As stated though, different districts and locations will have different densities of NPCs. And as Brandoid said, I would expect there to be performance options affecting NPCs as well, though we don't know that for certain yet.