I have a simple solution for this - although I'm sure there's a better way.
Firstly, I add an extra column - my counter. Then in the first row I insert the number <1> and in the second I insert the number <2>. I then select both these cells together, move my cursor to the bottom right hand corner and drag down to the end of the list. On releaseing, all the remaining cells in the counter column will be filled. Finally, I select the 'Number of items' cell for calculations and then <Hide> the column