Deauthorization

Exactly, nothing to worry about. Your locally stored media is unaffected by authorization. It is needed to play back or transfer protected media. Music purchases since late 2009 are DRM free, but audiobooks and video content usually have DRM.

tt2

There is no available list of authorized computers, you can keep your own. You can only authorize computers that you have physical access to. They don't need to be authorized if you're not using them.

See Authorization vs. association - Apple Community for further information.

tt2

pkBrooklyn wrote:

So I can deauthorize all and then go back and authorize the ones I want with no loss on the one I am using now?

Yes, although if you are an Apple Music subscriber it might be best to switch to a new empty library while you deauthorize and reauthorize to make sure any locally downloaded Apple Music content is unaffected.

Do I need to authorize my iPhone and iPad too?

No. Authorization only applies to computers. A computer must be authorized when transferring protected media to a device.

tt2

You don't need to, but see Use multiple iTunes libraries on PC - Apple Support. On a Mac you press and hold down Alt/Option rather than Shift to switch or create libraries.

tt2