Q: it is written here you can add max 10 devices to the iTunes match, but when you reach the 5 devices, you can't add more

You can have up to a combined total of 10 iOS devices and computers. As your iTunes account must be authorised on a computer's iTunes to be able to associate it, and your account can only be authorised on up to 5 computers at a time, then at most 5 of the 10 can be computers. But you should be able to have, for example, one computer and nine iOS devices.