Since the systemic approach deals with forms, structures and immanent logics instead of specific content, there is basically no composition within which it is not possible to work systemically.
Up to and including systemic organizational and management consulting, there is no general upper limit to the number of participants who can be involved in a cooperation.
Me personally, however, I limit myself to working with very small systems (individuals) as well as with very small groups (couples/families) or even small groups (up to a maximum of around 10 people).