Short answer - Alot! Its pretty hard to count how much germs a dog has, "germ" is just a widely known term for bad bacteria, which infects spreads and repeats with whatever it touches. The amount of bacteria would depend on many things, if the dog is sick, if it just went to the dog park, if it was just bathed, and if it recently ate. This is any type of bacteria of course not just bad, There's bacteria in your stomach right now, helping you digest any food. The point is though, is that the germs a dog has is like asking how many stars are in each galaxy , you can only give an estimation, but it will always be affected by the condition of the galaxy, or in the case the dog.
