How do I find Authorised Devices I have an iPod and iPad which I sync with my Mac.
My wife has has an iPod, iPhone  and iPad which she syncs with her Mac.

These are all associated with the same Apple ID.

I am planning to buy an iPhone, basically to replace my iPod.

I understand there is a limit to the number of devices, although I have not been able to discover the devices associated with my Apple ID.

http://support.apple.com/en-au/ht4627 has instructions which I have followed.

There are 3 computer authorisations listed, but there seems to be no Manage Devices on the Account Information screen.

NOTE We do not use automatic downloads or iTunes Match. I am only interested in apps
These articles on https://www.apple.com/support/itunes/authorization/ should help with authorization needs. In my understanding, you must de-authorize all devices and not just one. The limit is 5, so you still have room.