A 19 sided polygon is called :

An Enneadecagon

Here are the name of the first twelve polygons:

3 triangle

4 Square

5 pentagon

6 Hexagon

7 Heptagon

8 Octagon

9 Nanagon

10 Decagon

11 Hendecagon or Undecagon

12 Dodecagon

An Enneadecagon

Here are the name of the first twelve polygons:

3 triangle

4 Square

5 pentagon

6 Hexagon

7 Heptagon

8 Octagon

9 Nanagon

10 Decagon

11 Hendecagon or Undecagon

12 Dodecagon