The number of registered users or concurrent users that can use the chat is not limited in any way by JChatSocial.
The only limit to number of users is dictated by your server resources and available bandwidth.
If you experiment poor performance, site slowness, messages loss when a lot of users are using chat at the same time, you can reduce server load using the parameter available in chat configuration.
Reducing server load the chat will be less reactive, but server load and bandwidth usage will decrease.
The WebRTC video chat is a peer-to-peer technology that involves only end users without the server. This means that there are no limitations related to the server and to the chat application, in this case the only limitations can be due to the PC CPU performance, network bandwidth, etc. Keep in mind that having a video conference with many users may require a lot of CPU resources for each client involved, so there is a reasonable limit in this usage that must be tested case by case.