That's a factor of how many units are serviced by a set of stairs. On the same surface area you can have one large building with two staircases and one long corridor that splits the building down the middle, or you can have two separate buildings with a single set of stairs each and a better layout for apartments with more than one bedroom. The peak egress traffic will the the same for each case.
But all of this is unrelated to
> It does mean disabled residents are sort of screwed if there's a fire alarm and the elevators are shutoff--as a friend told me recently--unless the neighbors can help.