Most DVR / NVR systems have 4, 8, 16 or 32 channels, the number of channel ports on the back of the recorder is generally the number of cameras that can be supported by the DVR / NVR. Some NVRs can support additional cameras using a Power over Ethernet (PoE) switch. A PoE switch allows you to accommodate additional security cameras with the help of your router. It also allows you to install your cameras even further away from the NVR and reduce the amount of cables needed for larger properties.
Be sure to check your recorder's product specifications sheet to determine how many cameras are supported by your recorder and which cameras are compatible. Product downloads and documentation