I would like to deauthorize my apple ID from the 5 Pcs which i sold out for a long time

If you don't access to those computers to be able to deauthorise them directly on them you will have to 'deauthorise all', assuming that you haven't already done so within the last 12 months (if you have try contacting iTunes Support and see if they can / will do it for you) : Deauthorize your computer using iTunes - Apple Support

Why is there a limit to number of authorized devices? Why can I only wipe them once a year? Why do I have to deauthorize every single computer just to remove one old one?

Every part of this "solution" makes no sense.

Why is there a limit to number of authorized devices? Why can I only wipe them once a year?

Assuming that you mean 'authorise computers' (iOS devices are not authorised to an account) you will have to ask Apple, as far as I know they haven't said.

Why do I have to deauthorize every single computer just to remove one old one?

You don't, you can deauthorise an individual computer directly on it