Maximum connections limit for HTTP in IIS 6.0

I have an ISAPI extension, it works well in IIS 5.0. The current connections
web service can exceed 50000 in IIS 5.0. But in IIS 6.0, the current
connections for web service can't exceed 8704. In IIS manager, I have set the
site connections to unlimited. I also change the conncetion time out to 900
seconds. I even change MinFileBytesPerSec to 1. However, the connections
still can't exceed 8704. After the current connections reach 8704, the client
will get 10061 socket error.

I check Httperr.log, no error in log file.

Does anyone know how to resolve this problem. I have suffered this for one
week. Your help will be greatly appreciated.
Thanks in advance

James tu