how do manage, add and remove devices linked to my apple id?

Depends upon where exactly you want to remove / add them. If in the 'manage devices' section, to add them you need to do something that results in an association e.g. redownloading past purchases on them : View and remove your associated devices in iTunes - Apple Support

If the list of devices/computers where you are logged in, you need to log out of the account on the devices/computers that you want to remove, and log in (and stay logged in) on the ones that you want to add : Check your Apple ID device list to see where you're signed in - Apple Support

https://discussions.apple.com/thread/7729176?answerId=30889716022#30889716022 - In iTunes, go to your account page, look in the 'iTunes in the Cloud section, click on Manage Devices to see the list of authorized devices.