The number of concurrent connections is dependant on your serverhardware. The DR is nothing more than a share on a server that is accessed through your Agents configuration. Thus if you have a good network connection and server capacity the server may server more connections than when you have a mediocer Server with slow network connections.
This is a trick question, as you have to determine the type of Repository you are using...
UNC:Windows 2k3 and above only allow 5000 ephemeral Ports with out a reg hack. So lets say 1-2 ports per machine, puts it at something like 3500. Check out kb from MS (http://msdn2.microsoft.com/en-us/library/aa560610.aspx)
FTP: Depends on the Web Servers Max connections (But keep in mind the Windows ephemeral issues listed above, Linux is about 65k)
HTTP:Depends on the Web Servers Max connections (But keep in mind the Windows ephemeral issues listed above, Linux is about 65k)
SuperAgent:Depends on the Version of the Agent - MA 4.0 P3 and MA 4.5 resolved an issue were there was a Max of 5 connections. SuperAgent repositories now support up to 200 systems waiting to connect for download
ePO Server / Agent handlers: 250 total http connections (so this number could be smaller if you have machines performing normal ASCI connections.) Also this is configurable via the HTTPD.CONF, but we do NOT recommend changing it.