Limiting viewers

Yes. If you simply want to limit the number of connected clients, you can inspect the length of the application.clients array

application.onConnect = function(client){
if(application.clients.length >= 50){
application.rejectConnection(client);
}
}

If you want to count only specific users (i.e viewers vs. publishers) you'd need to keep track of your users individually and apply your logic in the onConnect method..