To use an ordinal counter, like this:

Welcome, you are the 120th visitor since September 12, 1997.

The code is:

Welcome, you are the <!--#exec cgi="/cgi-bin/counterord"--> visitor since September 12, 1997.

To use a numeric counter, like this:

120 visitors since September 12, 1997.

The code is:

<!--#exec cgi="/cgi-bin/counter"--> visitors since September 12, 1997.