That depends upon the original image. Most images in common use are 8 bits (uint8) per color component, 3 color components per pixel, so 24 total bits per pixel. If you had 0.1870 bits per pixel instead, your compression ratio would be 24:0.1870 or about 128.3 to 1
Some kinds of image have only 1 color pane, and some kinds of images use more or fewer bits per component.
imfinfo() can be used to extract information about an image file. In particular look at the Bitdepth field.