Adobe Stratus, the hosted RTMFP rendezvous service, can handle a very large number of simultaneous users. it is limited only by the number of computers we currently have in our cluster, and we can increase its capacity by adding more physical computers.
there is no inherent limitation to the number of peers any one Flash Player application can have. there is a default limit of 8 1:1 NetStreams that can be accepted by a publishing peer, but that number can be changed in ActionScript. the more realistic limiting factor to the number of streams that a peer can handle is the network bandwidth available to that peer.