The moon is about 30 arcminutes across (half a degree) [0], which is considerably larger than the area of the Extreme Deep Field. But the estimate of the number

The moon is about 30 arcminutes across (half a degree) [0], which is considerably larger than the area of the Extreme Deep Field. But the estimate of the number of galaxies is okay. The Hubble Ultra-Deep Field (of which the Extreme Deep Field is a part) is one thirteen-millionth part of the full sky. [1] The full sky is about 41,253 square degrees in size. [2] I think the HXDF is one 32-millionth of the full sky. If so, multiplying 32 million by the 5500 galaxies in the XDF gives on the order of 200 billion galaxies, as you said. [0] https://en.wikipedia.org/wiki/Angular_diameter https://en.wikipedia.org/wiki/Angular_diameter [1] https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field [2] https://en.wikipedia.org/wiki/Square_degree https://en.wikipedia.org/wiki/Square_degree