The passing branches are the same regardless of the number passed, just if there are more arguments the list is longer: brs(“foo”, b4, b6) with two, brs(“foo”, b4, b6, b2, b2, b3, b8, b0, b9, b3, b11) with ten.
All inbound arcs must carry the same number of operands, so if B passes 10 to C then A must also. Consequently C sees the same belt structure regardless of how or from where it received control.
Perhaps you are thinking that C will only need so many from B and fewer from A, and can tell the difference based on the values of the common arguments. Such a situation would be analogous to a VARARGS function call. This notion does not exist in common languages and is not directly supported by the Mill, although in principle the specializer could supply dummy arguments for A->C to fill out the branch.