There is no limit if streaming Internet Radio. However, some UPnP servers limit the number of clients that can be connected. This limit varies depending upon the server used (usually between 5 and 10).
Also of course your network may run out of bandwidth if you are using high bit rate uncompressed or losslessly compressed files. Ethernet is a better choice than Wi-Fi if you intend to use high bitrate file types (above say 500kbps).