The Hubble ultra-deep field image is a photograph of an area of sky equivalent to a 1mm x 1mm piece of paper held a metre away from the eye (one thirteen millionth of the entire sky). It contains an estimated 10,000 galaxies.
If galaxies were uniformly distributed, and the HUDF were representative of the entire sky, that would mean there were approximately 130 billion galaxies, which falls between estimates I have read of between 100 and 200 billion as the total number of galaxies.
However, my understanding is that galaxies occur in clusters, which would seem to imply that some areas of the sky should be more densely populated than others.
So is the HUDF representative of the entire sky, or are some areas much more and much less densely populated?