An n by n square will have n edges on the top row, n edges on the bottom row, n - 2 edges on the left side and n - 2 edges on the right side for a total of n + n + (n - 2) + (n - 2) = 4n - 4 edges.
For a 20 x 20 square, there will be 4(20) - 4 = 80 - 4 = 76 edges.
The border number will be 76.