There is generally no restriction on the number of lines and extensions that you can connect. However, there are many factors that work together to determine the overall capacity of your system. You can build an expensive and powerful PBX and load it down by using a few heavy features so that it can't handle many calls - or you can build an inexpensive and weak PBX and connect it to lots of extensions that are only used for simple calls. Because of this flexibility it is not easy to make general comments about PBX capacity. In general you will want to determine your needs and build a PBX that fits both your requirements and your budget.