The problem of photographic enhancement is usually not one of “making pixels” (scaling up). A photograph actually has a lot of pixels (a real one, that is); the average size of a silver grain on film is about 1 micron. A typical 35mm negative is 24x36 mm (don’t ask). Assuming you scan the film at the full resolution of the grains, you would get 24M x 36M pixels, or 864M pixels. Even if the region of interest doesn’t fill the frame, you will generally have plenty of pixels for the subregion (e.g. license plate).
The problem is that the lens system probably wasn’t focused exactly on the subregion (license plate), so you have to sharpen the image to enhance it. There are many techniques for sharpening, most of which use a convolution kernel. This is a matrix operation in which a pixel’s value is changed by applying postive and negative offsets computed from nearby pixels. For example, the “unsharp mask” operation is a convolution kernel. It is amazing how much contrast and detail can be recovered from an out-of-focus image using this technique. Often you really can read the license plate.
P.S. In case you’re wondering, there are devices which can scan at micron or even submicron resolution, which is two orders of magnitude beyond your typical flatbed scanner. My company Aperio makes instruments called ScanScopes which are used for scanning microscope slides; the resulting images can have a resolution of .25 microns (approx. 100,000 dpi). A typical microscope slide has a tissue area of about 20mm x 15mm, so the resulting digital images are around 100M x 60M pixels. That’s a lot of pixels.