Every checkbox tends to have a value, so counting them basically gives you the number of records. If the checkbox is checked (True) the value is -1, whereas if the checkbox is empty (False) the value is 0. One way to count how many of the values is true is to SUM the checkbox field, then take the absolute value, or simply (-SUM(checkboxfield)). Make this the data source for the control in your report or form.
-Larry