Unless something has changed since December, or CEO Mariina Hallikainen misspoke/was misunderstood, the million limit should just be residents and tourists. That's the way I read her statement from a previous interview, anyway.
RPS: So we followed Anna around. You believe you’ll be okay tracking a million people like that, from homes to jobs to other places, around the city? You can quite happily have a million different people moving about?
Hallikainen: Well, they aren’t all walking about at the same time. Some will be at their work. Some will be in their houses. There will be a million people possible, though to be honest that’s both tourists and citizens, not all of them are residents. Yes, you can have a million people in your city, you just can’t see them all on screen simultaneously.
RPS: But they still exist.
Hallikainen: Oh yes, it’s just about rendering and the technical limitations. That’s also the hard limit. There will not be a million and one.
Source:
http://www.rockpapershotgun.com/2014/12/17/cities-skylines/