At the Japanese contest, Peter said there is a closed form solution for the maximum number of walls a legal micromouse maze can have.
His suggestion was to draw out mazes in increasing size and the pattern will emerge. Also, the rules say every post, except the center post, will have at least one wall attached.
The number of walls for an N x N maze with at least one wall is:
4*N + (N-1)^2
The first term is for the outer perimeter and the second one is for at least one wall attached.
You need additional walls if you want spares or want to block off areas. I don’t have a good rule of thumb and am guessing 10% overage is probably not a bad idea.