Question

An authenticated user can add up to how many computer accounts to the domain, by default

Answer 1

By default, 10 computers can be joined to the domain by both users and administrators. As long as a user is authenticated against the Active Directory, he or she can add up to 10 computers to the domain.
While this one posses as an advantage for smaller companies, it is not a desirable feature for bigger companies since they have to control more tightly who can add machines to their domain.