What is a 16 sided shape called?

In mathematics, a hexadecagon (sometimes called a hexakaidecagon) or 16-gon is a sixteen-sided polygon.

Hexadecagon - Wikipedia

3 = triangle or trigon

4 = quadrilateral or tetragon

5 = pentagon

6 = hexagon

7 = heptagon

8  =octagon

9 = enneagon

10 = decagon

11 = hendecagon

12 = dodecagon

13 = triskaidecagon or tridecagon

14 = tetrakaidecagon or tetradecagon

15 = pentadecagon

16 = hexadecagon

17 =  heptadecagon

18 = octdecagon

19 = enneadecagon

20 = icosagon