How does the spatial resolution of an image relate to the number of pixels in a unit of length?

Yes, "the spatial resolution of the image IS independent of the number of pixels", yet..... once you determine the spatial distance one pixel represents then it is dependent.

You have an aerial image of your city, just a jpg or tiff.... the pixel count of the image is 300x500. This is your pixel resolution. If you want your spatial resolution you have to know the distance represented by each pixel. If a pixel accounts for a 10x10 KM square (always squares for the image) then you know your image has a rectangular distance of 3000x5000 KM. If each image pixel was 3x4 miles (the image pixel is always square, its representation does not have to be) then you know the image is 900x4500 miles.

Again, this is true "the higher number of pixels per unit of length in an image, for example, will make for higher spatial resolution of the image"

If you took that same picture of the city over again but upped your cameras resolution to 600x1000, now each pixel represents 5x5 km or 1.5x2 miles. So you will get a better spatial resolution with an increases image pixel count.

Theres a lot of variables that can change this....if you took the image at a resolution of 300x500 and then decided to cut each pixel into 4's, you would have a 1200x2000 pixel image with alot of repetitive pixels because the new image of 1200x2000 was interpreted opposed to taking a fresh image with the increases pixel rate. So in short you made each pixel, 4 pixels, all representing the same data the sole pixel once represented.....where you switch the resolution before retaking the picture and each pixel will be unique to what the camera saw.

Feel free to write me if you are still confused and would like clearance... This type of image work is what I do for a living.