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Any regular polygon with an even number of sides must have parallel sides.
A circle intersected by a pair of parallel lines.
A regular polygon with an even number of sides.
Infinitely many. Every regular polygon with 2n sides has n pairs of parallel sides. Then there are irregular polygons which can have parallel sides. And there are also irregular shapes which have parallel sides.
Their are 4 pairs of parallel sides on an octagon. In fact, for any regular polygon with an even number of sides, there will be # of sides / 2 number of pairs of parallel lines.