Transfer iTunes Store purchases to your Mac
In the iTunes app on your Mac that you’re transferring items to, choose Account > Authorizations > Authorize This Computer, then click Authorize.
Connect your device to your computer.
Click Transfer Purchases in the window that appears, or choose File > Devices > Transfer Purchases From [device].
You can play iTunes Plus songs on any number of computers; you can play other iTunes Store purchases on up to five authorized computers.