There are two apps to access OverDrive from your device: OverDrive or Libby.
Check-out limit: 15 at a time. These titles don't count against the number of physical library materials you have checked out.
Holds: 15 at a time. You have 3 days to check out a held title that is ready for you. If you use the OverDrive app, set your account to check out holds automatically, once they become available. This happens by default in Libby.