You cannot answer this with a normal shape. Any shape given would have less an infinite sides.
The exception to this would BE the shape with infinite sides and therefore by the same token, an infinite perimeter. So, the answer would be a "Fractal"
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An octagon.
A circle has infinite sides. Otherwise it's a hecatommyriagon with 1000000 sides So yeah.
a shape with ten sides
Octagon.
There is no 'most' - if you had enough time, you could draw a polygon with a billion sides, or a billion billion...... Then you could still always add more sides.