One, Two, or More
Since Yorkies are so small it may seem like a good idea to bring home more than one. After all, they can play with each other and burn off some of that terrier energy. They can keep each other company when you have to go out. And two will be twice as amusing for you, right? Not necessarily.
The Downside to Multiple Dogs
The main problem with bringing two puppies home together is that they will bond strongly with each other, perhaps to the detriment of the canine-human bond. You will have to spend extra time with each pup separately to ensure that they become comfortable around humans and learn to be apart from one another.
While two pups can entertain each other, they can also encourage each other in bad behavior. If one pup is in a very active, playful mood, the other one will likely join him in racing around in circles, even if it's the middle of the night. If one is whining to be let out of his crate, the other will likely whine along; the squeaky symphony of puppy voices is too much for even the heaviest sleeper to ignore. Also, unless you are supervising them constantly, you won't know which puppy left the puddle on the living-room carpet or chewed the leg of the kitchen table. You need to be able to identify which puppy exhibits which misbehavior so that you can rectify the problems with more training.
Another issue to keep in mind is cost. Obviously, two dogs mean double the expenses, double the time, double the grooming, etc. Two dogs is a lot for a family to take on, and it's an especially big challenge for a single person or a working couple. Be sure that you can handle the cost and time commitments before you bring home two (or more) new pets.
The Plus Side to Lots of Pups
Clearly, the biggest benefit to twice the dogs is twice the fun! And if you get three or more dogs, they will be an adorable group of friends that can play together, sleep together, and take walks together. You'll certainly never be lonely with multiple Yorkies in the house. Additionally sleeping in groups will remind the pups of when they were with their mother and littermates and will comfort them at night; this will hopefully mean less whining and more sleep for you. And one person can certainly walk two or three Yorkies, given their small size. So, even with multiple dogs you will only have to take them on one group walk.
Unless you have a reputable breeder as a mentor, don't buy two Yorkies with the idea of breeding them and recouping your investment. Yorkshire terrier litters are generally small, and veterinary bills can be large. You may find yourself losing more money in the venture than you originally spent on the dogs in the first place.