Well, with one dog, you can devote so so so much time to that one dog and his/her training.
With one dog you only have one food bowl to fill and consequently only one dog to clean up after.
One dog to walk. One dog to travel with or find a pupsitter for if you go somewhere.
One dog to vaccinate, worm, etc.
That being said...
If your dog does well with other dogs, being able to play with other dogs is so good for them. Another dog can give them the kind of play and communication that we can't possibly give them because we have an anatomical disadvantage...we're humans, lol! That doesn't mean that a single dog is not happy, by any means, don't take that the wrong way.
Having two dogs means being able to teach double-dog tricks--my favorites! (Although that is certainly NOT a good reason to get another dog, lol!

)
Having two dogs means
always having a distraction ready when you're working on advancing certain tricks.
I can say that having 3 dogs and training all of them for various things is a lot of work. And a lot of food. We have 4 dogs, 3 being mine and 1 being my mom's. It's a housefull, lol! But, I love it. I don't remember what it's like to only have one dog.
If you're considering it, I think it's a great idea to foster. That way, you can see if you are ready for another dog and see what it takes to raise two dogs, and you'll be helping a dog find a home.

If you decide one dog is plenty for you after the dog is adopted, then you're back to your one dog and you've helped a dog find a home.