It really depends on what you mean by rectangular figure. If you mean a rectangle in 2D space, then you need at least 4 points to represent it but that's not enough for drawing the shape.
When you're drawing a shape, you need to specify its triangles. Let's say you have a rectangle with the points [0, 0], [0, 2], [1, 0], [1, 2]. To draw this you need to specify at least two sets of dots. First one would be something like [[0, 0], [0, 2], [1, 0]] which would draw the first triangle. The second one would be something like [[0, 2], [1, 2], [1, 0]] which would draw the second one. Notice that I'm going clockwise when representing the triangles. This really doesn't matter much so long as you draw them all the same way. If you draw one clockwise and the other one anti-clockwise, then you'll only see them on the opposite sides of the shape.
Anyway, this 2D rectangle is represented by two triangles and four points in space. That's would make six matrices in total. Seven, if you count the overall matrix that is the shape itself.
In 3D space it's a bit more complicated. You now have 8 points in space, and you have to define 6 faces for the rectangular prism. Each face will need two triangles, which means 12 matrices. In total, you would have 8 for points and 12 for triangles, which makes 20 matrices.
Of course the correct terminology is vertex and vertices but they're still represented as matrices, or rather arrays, in game development.