Rather should be titled <R18> Clarification
Nothing is said about pistons, and the general assumption regarding VEX parts is that you may use as many VEX parts as you can, unless rules state otherwise. Thus you may use an unlimited amount of pistons, but you are limited to two (2) air reservoirs. The purpose of that is to limit you to the pressure provided by these reservoirs, as well as the normal working pressure in the pistons and tubing. Pneumatic systems lose pressure every time you actuate a piston, so keep that in mind.