How detect how much a server support connection

I have a downloader program in MFC/Win32 that downloads a file at multi-connection but on some sites when connection count is 8 the server returns 503 error but when I decrease connection count to 5 download works fine? Also I set INTERNET_OPTION_MAX_CONNS_PER_SERVER and 1_0 server value to 64 How to solve this problem? How detect the server supported connection count? Thanks