In general, none.
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There is no such word as a dectagon. And since it could refer to an octagon or a decagon (or possibly something else), it is not possible to give a sensible answer. Please correct your spelling and resubmit the question. If either octagon or decagon, they need not have any parallel lines.
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.
Ten.
the answer is parallel lines