=MAX(SI(Y(F:F="llave1";P:P=10);L:L))

sesionesMes = WorksheetFunction.Max( _
                            WorksheetFunction.IF( _
                                    WorksheetFunction.And(Columns(7) = llave, Columns(16) = mes), _
                                    Columns(12) _
                            ) _
                        )

sesionesMes = WorksheetFunction.MaxIfs(Columns(12), Columns(7), llave, Columns(16), mes)