Puppies should be let to stay with their mothers and siblings for at least 2 months, ideally 3 months. If they are in a good home with a good mother and with people that socialize with them, also letting some visitors see and handle the puppies, the puppies will automatically be well prepared to meet the world.
Puppies will not have problems with meeting other dogs. The new owners of the puppies should take the puppies to meet and see all kinds of people, places and other animals little by little. The owner should be like a loving companion and never punish, force, or scare the puppy. Puppies will learn everything, but easy does it. They should be let to meet many other dogs off the lead when possible.