Counting the number of checkboxes...

[Gerry]

Late to this party but I would have thought that the simplest solution was to have a field on one of the cards that has 0 in it at the start. 

Then in each check box button have a script that simply adds 1 to the value in that field when the check box is clicked on, and subtracts 1 from the field when the check box is clicked off. Whenever you need to know how many of the check boxes are on just get the number from that field. 

Gerry