Puppies need to stay with their mums until at least 6wks otherwise they run the risk of having problems with other dogs.
Then from about 8wks to 14wks you have a window of opportunity to introduce them to as many things as possible as anything they meet these weeks they are unlikely to be afraid of for the rest of their lives. The puppy should see cars, bicycles, trains and city lights and sounds, it should see open fields, cows, sheep, horses, pigs and the countryside, it should meet as many new people a day as possible, wearing hats, carrying umbrellas, using walking sticks, and especially children and the noises they make, it should be exposed to household sounds like the TV, dishwasher, Hoover, etc. basically anything you can think of expose the puppy to it.
Please talk to the breeder about this and be weary of him putting you off. Early socialisation is crucial for having a well adjusted dog. And before anyone shares their story of their under socialised dog that is doing fine now, of course there are exceptions to the rule and of course the risk of future problems is statistical and not absolute.