Rule of 7's for socialization
Some people use 7, others use 12.
There are new articles which state over socialization could create additional issues.
http://www.collared-scholar.com/more-harm-than-good-3-reasons-why-i-never-socialize-my-puppies/
I agree with early socialization, to prevent fear based responses in dogs. Over socialization in older puppies may create focus and distraction issues. Find a middle ground you're happy with, but keep in mind, that under socializing can create a fearful dog as well.
For my puppies, I make sure they have experienced the following (at the very least):
Interpreted from: Aggression in Dogs: Practical Management, Prevention & Behaviour Modification
- Been in 7 different, safe locations
- Eaten from 7 different containers
- Met at least 7 different people
- Ridden at least 7 miles in a car
- Been in a crate at least 7 times (more is better)
- Played with 7 different kinds of toys
- Been exposed to 7 different contexts
Been played with or taken somewhere alone, without mom or litter mates, 7 different times