When they were still alive I would let our two labs take turns in me every afternoon we were alone, this would go on for hours as they would each take 4+ turns and leave me quite messy. Starting in college and going in for years I would service other dogs and sometimes strays, at 2-4 per night depending on how close they were to each other.
The most I ever had in one session was a gang bang with 8 dogs, that took me in a train, followed by the owners, and then the dogs again, followed by the owners again.