A size chart tells you a charger holds 666 g or 2 kg. It does not tell you how many portions that is, which is the thing anyone actually wants to know.
The honest answer is that it depends on four things, and any supplier quoting one fixed number is guessing. What you are making: a stiff whipped cream uses gas differently from an airy foam or an infusion. The dispensing equipment and how it is set. How full each charge is run. And how often the system is opened, because every reset costs gas.
What you can do is measure your own kitchen. Note how long one unit lasts across a normal week, then read the sizes against that. A bench getting a week from a 666 g unit is comfortable. A bench getting two days from one is paying in interruptions and should look at 2 kg or 4.33 kg.
That is also the real argument for the 5 kg and 10 kg tanks. They are not about capacity for its own sake; they are about not stopping mid-service to change something.
Whatever size you settle on, confirm it against the current manufacturer instructions for your regulator or siphon. Products are supplied for lawful culinary and food-service use, within the applicable regulations.