By default the number of simultaneous connections to a HTTP 1.0 server is 4, to a HTTP 1.1 server is 2.
To change these limits, use Regedt32 to navigate to:
On the Edit menu, add the following REG_DWORD value names:
MaxConnectionsPerServer - The number of simultaneous requests to a single HTTP 1.1 Server. The default is 2.
MaxConnectionsPer1_0Server - The number of simultaneous requests to a single HTTP 1.0 Server. The default is 4.
NOTE: Increasing these defaults is a violation of the HTTP protocol, and should only be performed if absolutely necessary.