Knowing a unit holds 666 g or 2 kg does not tell you how many portions come out of it. That is the number kitchens want, and no supplier can quote it honestly as a single figure.
Four variables decide it. The preparation itself, since a firm whipped cream consumes gas differently from an aerated foam or an infusion. The dispensing equipment and its settings. Whether each charge is run out fully. And how often the system is opened, because every reset costs gas.
The usable method is to measure your own bench. Track how long a unit lasts over a normal week, then read the catalogue against that figure. A week from a 666 g unit is comfortable. Two days from one means the cost is being paid in interruptions, and 2 kg or 4.33 kg is the answer.
That is also the honest case for the 5 kg and 10 kg tanks: not raw capacity, but service that is not paused to change a unit.
Verify whichever size you choose against the current manufacturer instructions for your regulator or siphon. Products are supplied for lawful culinary and food-service use within the applicable regulations.