A triangulation of an n-sided polygon should have n−2 triangles. So the question turns into why you got 141 triangles from 144 points.
If you play around with a few examples, you quickly discover that every triangulation of an n-sided polygon
has n−2 triangles. You might even try to prove this observation by induction. The base case n = 3
is trivial: there is only one triangulation of a triangle, and it obviously has only one triangle! To
prove the general case, let P be a polygon with n edges. Draw a diagonal between two vertices.
This splits P into two smaller polygons. One of these polygons has k edges of P plus the diagonal,
for some integer k between 2 and n − 2, for a total of k + 1 edges. So by the induction hypothesis,
this polygon can be broken into k − 1 triangles. The other polygon has n − k + 1 edges, and so
by the induction hypothesis, it can be broken into n − k − 1 tirangles