​How many NVRs can 1 camera be added to?

It depends on how many video streams the camera has.Each NVR will occupy 3 streams(Main/Sub/Third Stream)of 1 camera. 

Generally, 1 camera has 20 streams, which means it can be added to 3 NVRs totally. But for some old firmware cameras, they only have 4 or 8 streams.