This is really two separate questions: how much coverage can you get with a single hub, and what can you do if a single hub is not enough.
In most cases if it’s just one residential building, even a large multi floor Villa, one hub will be enough if it is placed centrally both horizontally and vertically providing the building is not constructed of concrete or Adobe. If it is constructed of materials that reduce the strength of wireless signals, then it becomes more challenging. The following thread (this is a clickable link) goes into more detail.
And this one:
If one hub is not enough, the SmartThings architecture does not support adding a second hub and coordinating them. You are limited to one hub per “location.” It is possible to define multiple locations in one building, such as one per floor, but it does require some duplicated efforts. Most of those details are discussed in an article in the community created wiki on automating an outbuilding. Even though that is somewhat different situation since it assumes there are two buildings, Many of the same technical issues apply.