Quality is delighting customers
No hard rule for that, but between 5-10 is good. The more team members you have, the more communication channels you will have. If you feel more work/team require to finish the projects then follow scrum of scrums. Start with 5, that is Ideal but don't force that thing, see how many can efficiently complete the stuff in sprint, then do +,-.
The Team: The scrum development team size is generally size of 5-9 peoples with self-organizing and cross-functional skills who do actual work like Analysis, Design, Development, Testing, Documentation etc.
You can read more information on blog - http://www.softwaretestingclass.com/agile-scrum/