You can have ten devices (no more than five computers) associated with your Apple ID for purchases at one time.

When you try to associate another device that was previously associated, you might see a message that says, "This device is associated with another Apple ID. You cannot associate this device with another Apple ID for [number] days." Depending on when you associated your device with your Apple ID, you might need to wait up to 90 days to associate the device with another Apple ID.

This list of associated devices is not the same as the list of devices that you're currently signed in to with your Apple ID. You can see where you're signed in on your Apple ID account page.