I think it’s important to analyze the reasons you are considering getting another dog. Is it for companionship for your dog, or for you? Or something else?
I don’t have a dog of my own right now because I am 23, and although I long for the companionship of a dog, I know that realistically I don’t have the time to train one, wouldn’t want to be tied down to a walk every day, live in an urban city, and don’t have the finances in the event that a vet visit would be necessary. Which brings me to my point, which is that I feel like a lot of people get dogs as accessories and don’t take the time to train or exercise them and just get them because they want one (or more than one).
If you are already giving your current dog everything it needs, meaning you’ve taken the time to train it when it’s young and walk or exercise it daily, and give it plenty of love and affection, then having another might not be a problem for you. I personally believe that one dog is enough, but if you have the time and energy to actually CARE for the dogs rather than just keep racking them up like tally marks like some people do, then I don’t see anything wrong with that.
I agree with Gail that in a more rural area it’s probably easier to have more pets, but in the city it just seems like two or three untrained dogs living in a one-bedroom condo who only get one 30-minute walk a day are not really living a happy life, though their owners probably are.