How could I know how many video streams a network camera is capable to transmit simultaneously via ONVIF and if there are any restrictions? I've search in every ONVIF's specs and nothing shows up. So far I've been able to know it through the manufacturer's web page or manuals.
Number of Unicast video streaming is generally provided by the manufacturer and also on the different types of resolution it can stream. If you meant to say number of simultaneous unicast user, then it is more or less 20. You should ask the manufacturers, they are very helpful at it.
Secondly, what is RTSP(real time streaming Protocol) it is a multi-cast protocol. What is a Multi-cast Protocol? it is for Net Streaming of Net Radio or IP Video/Audio to multiple Clients throughout the world. Is there a upper limit of this technology? No, as long as the bandwidth can support it.