Table.AddColumn(
        Source, 
        "b", 
        each List.Count(
            Expression.Evaluate("{" & Text.Replace([a], "-", "..") & "}")
        )
    )

    functions_text = 
        "[
            func_sum = (lst as list) => List.Sum(lst),
            func_combine = (a, b) => a & b
        ]",
    functions_record = Expression.Evaluate(functions_text, #shared),
    sum = functions_record[func_sum]({1, 2}),
    comb = functions_record[func_combine]("x", "y")

=ROWS(
    UNIQUE(
        FILTER(
            Таблица2[Ключ],
            LEFT(Таблица2[Исполнитель], 1) = "С"
        )
    )
)