Each machine has 2; one between the boiler and the top of the base, and one between the bottom of the base and the boiler flange. Sold individually. If you wish to replace both, you will need to order 2.
Note: Although you need the smaller P-4E on top of the base (or it will protrude out from the allotted space), Stefano prefers to use the larger spacer, P-4P, between the bottom of the base and the boiler flange as it offers a better “grip” due to the additional material.
Some folks call these parts a gasket, but they are just spacers to help prevent the boiler from slipping on the base. To access these spacers, you must remove your heating element and boiler flange, and you should replace the element gasket when reinstalling it—either the P-7 for brass and copper elements or the P-7ss for stainless steel elements. (See links below.)