10
One through each pair of vertices, and one through each pair of midpoints - of opposite sides.
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Ten.
A decagon need not have any lines of symmetry. It can also have 1 or 10 lines of symmetry.
A decagon need not have any lines of symmetry. It can have 10.
A regular decagon has 10 lines of symmetry. Each line of symmetry can be drawn through a vertex and the midpoint of the opposite side, as well as through the midpoints of two opposite sides. This symmetry reflects the decagon's equal angles and sides, allowing for multiple symmetrical divisions.
A dodecagon need not have any lines of symmetry. It can have a maximum of 12.