Berkay Dursun, who worked at IO Interactive for three years directly on crowd technology, commented on the leaked GTA VI gameplay and comparisons to Hitman. According to him, the dense crowds in Hitman and Assassin's Creed are technically different from the NPCs in the new GTA.
As someone who worked directly on this crowd technology at IO for 3 years, I can say that this crowd technology has nothing to do with the technology in GTA VI. In GTA, each character is simulated separately, and achieving simulation on such a scale puts them at least 10 years ahead of the industry. The closest game that performs simulation at this level might be Mount & Blade 2: Bannerlord, and yes, I worked on that game too, lol.
He explained that in Hitman and Assassin's Creed, many characters are not real NPCs but a "crowd" that follows predefined scripts and does not use AI. Real NPCs react to player actions and have their own behavior, which requires significantly more resources.
Dursun also stated that he could not disclose the exact number of real NPCs in 007 First Light, but the game "cannot even come close to GTA VI."