One data jack can support four analog phone lines, or unlimited VoIP phone lines.
For VoIP phones, one data jack can essentially have an unlimited number of phone lines associated with. By installing a 6 line phone and then adding on sidecars you can ensure that multiple lines will appear on the single data jack.
For analog/Centrex phones, in one jack you can install four phones on the voice port of the jack. For example, two phones, one fax, and a credit card machine.