A qustion about herb spiral

Hello all.
I have a customer that I wish to put a herb spiral in her garden.
I need to order the smount of stones for it (there is no avilabe stones in the area to use and she is kind of welthy so she wants to buy).
Any way, I cant calculte the amount of stons I need.
The diameter of the spiral will be 1.2m and the hight 1m.

So can anyone help me with the math?

Thanks!