The “pixel aspect ratio” is not a single number. It is a ratio between two numbers, the width and height of each pixel. So “1:1” or “100:100” would both mean the pixels are square.
But the ratio can always be expressed as “1:N” where N is a number that you calculate by dividing the Y-resolution by the X-resolution. Those two resolutions are often the same, because images often have square pixels.
The only images with non-square pixels I have encountered are fax images. But see also Pixel aspect ratio - Wikipedia .