I think it has 16 rectangles.
The window design has 6 small rectangles. 3 rectangles on top to the other 3.
1 2 3
4 5 6
1 = 1 big rectangle that encompasses the whole 6 small rectangle
6 = 6 small rectangles
2 = rectangles formed by 1-2-3 and 4-5-6
3 = rectangles formed by 1-4, 2-5, and 3-6
4 = rectangles formed by 1-2, 2-3, 4-5, 5-6
