When Seimitsu set out to create a new pushbutton, they went through many steps to ensure the perfect fit and performance.  With these washers, this allows you to set different MX Cherry switches to the proper height to optimal activation.  This washer comes stock with the SEPS series pushbuttons so this would serve as a replacement should you lose or damage your current set.  This piece is five times as thick as the metal washer that is also offered

These are measured as 0.2mm in height or 1/5 of the stock resin washer 

This comes in a set of 10 units

What is the correct amount of washers to use in each switch?  

The activation point for MX switches vary by type.   So depending on which switch you have, the number of either metal or resin washer(s) applied will vary.   You would have to test these yourself to see what the ideal activation point is