AFAIK, min-max normalization is to rescale the image pixel values to be between [0, 1].
The formula is,
scaled_img = (img - img.min()) / (img.max() - img.min())

In case of images ranging [0, 255]. img.min() = 0, img.max() = 255.
Thus, min-max normalization for these images is to divide by 255. (this assumes that image range is [0, 255])
Hope this helps!