First off, Hello and welcome to yt!
There are many factors you will want to consider. Do you already own yorkies or will you be looking for a good foundation? If you don't have any at the moment then I would say that's great because you can learn all you can prior to purchasing nice, sound yorkies for breeding. Purchase books, read them like 20 times each (lol
) and find an experienced breeder to help you. But to answer your question, females generally should be over a year old and have had their 2nd heat cycle before breeding. Yorkies can have anywhere from 1 pup up to several pups. I had a litter of 6 once. The amount of puppies is going to depend on the number of eggs the bitch releases along with how many were fertilized. Timing of breeding can play into this.