Hm, I agree that there's probably lots of fibs on this thread, but.... I do think the same dog, in his prime, can really mount and knot 2 to 3 times in a session, and probably 4 to 5 times in a day, spread out. Of course, that would likely be only after he went several days without being able to mate.
3 times every day for many consecutive days? I think that would be pushing it, but probably can happen in their prime.
The most interesting thing to me is how the dog will ask for it with his eyes and body language, and how the human lover is willing nearly every time he asks, if they aren't caught up with a more important task at that moment. The part of giving to his urges is really interesting, and apparently not all that uncommon.