I am looking for a list of the 20 USA cities with the largest number
of dogs.  In this list I would like to see the name of the city and
the corresponding number of dogs.

This is the closest I could find:  A formula from the American
Veterinary Medical Association.   The second site shows its use as
applied to US Census statistics for Los Angeles area.  At the bottom
of this site you can click to the Census website.  Note:  this give
dog-owning households, some of which will have more than one dog, but
in the largest cities probably not so many, perhaps averaged out by
lower dog ownership in major cities.

http://www.avma.org/membshp/marketstats/formulas.asp
http://www.laalmanac.com/environment/ev21d.htm

Another site estimated the US dog population at 60,020,000.