A listing states that a unit holds 666 g or 2 kg. It does not state how many portions that represents, which is precisely what a pastry kitchen needs to know.
Four factors govern it, and a supplier offering one fixed figure is guessing. The preparation itself: a firm chantilly consumes gas quite differently from an aerated foam or an infusion. The dispensing equipment and its settings. Whether each charge is run out completely. And how frequently the system is opened, since every reset carries a cost.
The practical method is to measure your own bench. Observe how long a single unit lasts across a normal week, then read the catalogue against that. A week from a 666 g unit is comfortable. Two days from one suggests the cost is being paid in interruptions, and 2 kg or 4.33 kg addresses it.
That is equally the honest case for the 5 kg and 10 kg cylinders: not capacity in itself, but service uninterrupted by a unit change.
Confirm whichever size you select against the current manufacturer instructions for your regulator or siphon. Products are supplied for lawful culinary and food-service use within the regulations in force.