You can only run one live stream per board. You are allowed up to four boards per server (assuming you have the slots available).
In addition to the live stream, you can have "any number" of other multicast streams from saved files. The actual number of additonal will depend on the type of file (.AVI, MPEG1, MPEG2, H.261, etc), and whether you want to leave room for on-demand transmissions.
Scott