How many floors can one set of elevators service?

[kenratboy]

For example, lets say you had a very tall residential skyscraper with only a few units per floor - how many floors can one elevator car/shaft service (as opposed to having one set for the bottom half and one set for the top half)?

Do you think 50 floors could be done?

Also, are there any rules of thumb for a residential tower as to how many elevators are needed to service a given number of units (for example, a 20 story building with 30 units per floor, how many elevators would be needed if all of them went to all floors)?

Thanks!