😏
You are right, my question is not clear enough.
Let me reformulate it in a concrete way.
I would like to get dynamically the amount of members of a given group.
My aim is simple: we have a specific group which holds all the registered privileged members who paid some money in addition of have simply registered. These privileged members can navigate the full web site without any AD and have access to specific services (same notion that "silver", "gold", ... members).
I simply want to display to all the amount of people who accepted to pay (ie. who accepted to become a privileged member).
And my idea was to create a simple block and use the template language to compute the given amount.
I agree that my initial question was more to request for some help on where to find any documentation. And I wanted to underline that I found no help nor documentation to drive me to the solution.