Airtime streams its output to an Icecast 2 or Shoutcast server – normally on the same machine.
The concurrent connections are dependent on the bandwidth available from the Icecast/Shoutcast server machine (if you see ‘unlimited’ - start laughing and look for the ‘reasonable use’ clause), the bit-rate of your stream(s), how powerful the server is, the amount of client connections set in the Icecast/Shoutcast config and lots of other stuff.
Sorry this is not a definitive answer - but its a complex question. There are some charts on the Icecast site.