Complete the analogy, in which each number counts the lines of symmetry of the shape named: Equilateral triangle : 3 :: Regular hexagon : ?

A regular hexagon folds onto itself about 3 lines joining opposite corners and about 3 more joining the midpoints of opposite sides, which makes 6 in all. The trap 3 counts only the corner-to-corner lines. 12 counts every line twice, once from each end. 9 is the number of diagonals of a hexagon, worked out as $6\times 3÷2$, and not its lines of symmetry.