A listing tells you a unit holds 666 g or 2 kg. It does not tell you how many portions that is, which is what you actually want to know.
Four things decide it, and anyone quoting a single figure is guessing. What you are making — a firm cream uses gas differently from a light foam or an infusion. The equipment and how it is set. Whether you run each charge right out. And how often the system gets opened, since every reset costs you gas.
So measure your own bench instead. See how long one unit lasts through a normal week, then read the sizes against that. A week out of a 666 g unit is fine. Two days out of one means you are paying in interruptions, and 2 kg or 4.33 kg is the fix.
That is the honest case for the 5 kg and 10 kg tanks too. Not capacity for its own sake — not stopping halfway through a service to change something.
Whichever size you land on, check it against the current manufacturer instructions for your regulator or siphon. Everything here is supplied for lawful culinary and food-service use, inside the regulations that apply.
