When freshmen move in starting Aug. 19 for McCarthy residents & 21st for remaining freshmen, is the availability of lockers at USC limited because they get filled up with student orders? And how long can items stay in lockers (3 or 15 days)?

Usually three business days

I don't know if availability will be limited. I do know that you have 3 business days after you receive the pickup code to pick up the item from the locker and after that the items are removed from the locker and returned.

USC Amazon Lockers can be used by students or the general public. I have never had any problems with grabbing things from there. 15 days usually. Just watch your Amazon account, it'll tell you by when it needs to be picked up.