A thickness of 1cm is standard and provides a close contact feel for the rider.
The thicker 2cm inserts are more often used for riders who are above 175lbs or when the horse is lacking topline and needs additional protection due to a prominent spine.
If you are unsure which saddle pad style you have, a Universal pad compared to a Shaped pad cut specifically to fit your saddle's model, please feel free to contact me and so we can best determine which option to select from the drop down menu.