getSelectedCount()
This method returns all rows which are currently selected in a Rich Display File grid widget that is configured for row selection. The return value will be zero if the grid is not configured for row selection.
A grid is configured for row selection if the "row selection" property is set. Also, getSelectedCount() requires the "selection field" property to be set.
Example:
var count = getObj("Grid1").grid.getSelectedCount();
alert("You selected " + count + " rows.");