How can I remove one or all computer(s) from iTunes Match?

Just wanted to add one of my Macs to my iTunes Match. Got the message that I already have the maximum allowed computers activated for iTunes Match and that I have to de-authorize one first.

(1) Where/how can I see what computers/iOS devices are currently activated?

(2) How can I deactivate one from the list?

(3) How many Macs and how many iOS devices are allowed with one iTunes Match subscription?