The listing says 666 g or 2 kg. It does not say how many portions, and that is what you actually want to know.
Four things change it, and anyone giving you one number is guessing. What you are making — a thick cream goes through gas differently to a light foam or an infusion. The kit and how it is set. Whether you use each charge right up. And how often it gets opened, because every reset costs you a bit.
Easier to just measure your own place. Watch how long one lasts over a normal week, then look at the sizes with that in mind. A week from a 666 g unit is fine. Two days from one and you are losing time to changes — go up to 2 kg or 4.33 kg.
That is the honest reason for the 5 kg and 10 kg tanks too. Not holding more for the sake of it, but not stopping in the middle of a busy night.
Whatever you pick, check it against the current manufacturer instructions for your regulator or siphon. Everything here is for lawful culinary and food-service work, inside the rules that apply.