In Jami, for video-conferences all clients are not directly connected to each others. This can't work, you can't ask everybody to stream and receives each video streams. Also, you can't ask each client to mix the streams together for the client.
You can see the video conferences in Jami more like a mesh between all participants of the call. Some nodes will mix the streams together (for now it's the device which receives the calls to merge together, they will need both good CPU & bandwith for sure) and the other nodes will only receives the mixed stream (and sends their stream)
So, for the host you can count 1Mbps/participant (to get a good h264 video stream) and the receiver the CPU & bandwith will be the same for a conference with 15 participants or 2.
We got a lot of conference with 15 people hosted by a x220 (and a lot with a P51, but it's a bit more powerful).