THe “What plan should I choose?” page mentions:
GitHub provides two types of plans: free plans and paid plans.
Both plans have the exact same features. They can have any number of public repositories, with unlimited collaborators.
That being said, too much of anything will be eventually detected and reviewed by GitHub in order to check if the service isn’t abused.
Update April 2020: as I explained in “Can a repository on GitHub be owned by 2 users?”, “GitHub is now free for teams“.
The number of public or private repo is now unlimited.