For example, I want to use air tanks to supply pressure to up to 18 pneumatic actuators (19475c01), but I don't know how many air tanks I should buy to supply the air pressure required by all of them. Does it have something to do with load? I suppose that I don't have too much load.
An air tank (at least LEGO low capacity ones) are not meant to directly supply actuators. Their main purpose is to smooth out the pressure of air flow produced by a pump (manual or motorized).