Stream video to MANY clients?

Yes it' s possible, you have just got to publish the stream in the instance of the app you want ( or just in rtmp://www.yourservername.com/yourappname ) and then every client that connects to the instance can play the stream, here is a small piece of code which should help you:

this is the code that create the stream (a live stream):

var nc:NetConnection;
nc.connect("rtmp://myserver/myapp/myinstance");
ns = new NetStream(nc);
ns.publish("stream_name", "live");

and this is the code with which you play back the stream:

var nc:NetConnection;
nc.connect("rtmp://myserver/myapp/myinstance");
var play_ns:NetStream = new NetStream(nc);
play_ns.play(radio);

then, if you want to add a webcam to your stream you can add this code when you create the publishing stream:

video = new Video();
video.attachCamera(camera);
ns.attachCamera(camera);

Hope I ve been useful, for a better understanding of these things i think you should look at the FMS docs and inside the FMS_INSTALLATION_DIR/documentation/
Bye!