I currently have four, with two seniors who enjoy their naps, and two youngsters who keep each other entertained. My dogs are fairly easy-going, like most bassets, and introducing new dogs to the household has not presented major problems. When I bring a new dog home, I usually crate the other dogs. I then bring the new dog into the house, and after showing the new dog the house a little, out into the backyard and into an ex-pen or fenced run. I let the other dogs into the yard, and all the dogs get acquainted through a fence under supervision. It's usually not a big deal to eventually open the run and let the new dog out to make friends, and most often, the new dog will follow the others' lead, in terms of using the dog door, etc.
As for how many dogs is too many dogs...obviously, finances are a major consideration, and then a person's available time and energy also need to be factored. At one point, I had five dogs, and I felt a bit stretched. However, I also know people who single-handed care for a dozen or so bassets, so it's an individual thing.
I wasn't really looking to add another dog to the household when we adopted Macy last summer, but she's been wonderful for Caper, providing her with a young playmate. Until she came, poor Caper was a kid with a bunch of grouchy seniors for company. I'll bet your younger dog would appreciate a playmate, too.
