The easiest way is to assign a linked cell to the checkbox and then look at that
linked cell.
So if you have 10 checkboxes hovering over A1:A10 and have 10 linked cells in
B1:B10, you could use:
=countif(b1:b10,true)
Depending on what checkbox you used (from the Forms toolbar or from the Control
toolbox toolbar), the way you change that linked cell varies.
Forms toolbar checkbox
Rightclick on each checkbox
Choose Format|Control
then Control tab
Then type it into the cell link box.
Control toolbox toolbar
go into design mode (another icon on that control toolbox toolbar)
Rightclick on each checkbox
Properties|LinkedCell
and type it in.