One option is to create formula column with following formula to get 1 when column is selected.
Selected()
If you only care about total you can then count them from that formula column or possibly use Col Sum directly in the formula
Col Sum(Selected())
See Special Formula Columns for more fancy uses for special formula columns like that.
Other option is to use Row State Handler (or Graphic Script) with something like:
N Items(Current data Table() << Get Selected Rows)
-Jarmo