and what do we call a 12 sided  figure 13,14 and so on till 20

12 sided figure of polygon is known as dodecagon.

Similarly 

13 sided = tridecagon or triskaidecagon

14 sided = tetradecagon or tetrakaidecagon

15 = pentadecagon or pentakaidecagon

16 = hexadecagon or hexakaidecagon

17 = heptadecagon or heptakaidecagon

18 = octadecagon or octakaidecagon

19 = enneadecagon or enneakaidecagon

20 = Icosagon