If I understand you correctly then I think this will do what you need:
Put a textbox in the group header with a controlsource of =1 and RunningSum set to OverGroup. You can set its visible property to False if you don't want a running total shown in the report. Then in the report footer add a textbox with a controlsource of =textbox1 (where textbox1 is the name of the running sum textbox). This will then give you a count of how many Group Headers there were.
Hope that helps.