I am not exactly sure what you mean by cell number. If you want to get the current click cell when a row is selected you can subscribe to the SelectedCellsChanged event of the RadGridView and use the CurrentCell property of the grid. This property represents the current cell.
var selectedCell = gridView.CurrentCell as GridViewCell;
If by cell number you mean the row and column index you can get the column index from the DisplayIndex property of the column of the cell. As for the row index, you can use the ContainerFromItem and IndexFromContainer methods of the ItemContainerGenerator property of the RadGridView. Check the following code snippet.