Avast account

Good morning!
I wonder if there is a maximum limit of devices that can be added to avast account

Where are only devices that use the free version of avast

maybe you find some info in FAQ section https://www.avast.com/en-eu/faq.php?q=account

You may be interested in the Avast for Business (Free) version.
https://www.avast.com/avast-for-business

is permanently free regardless of the number of computers?

As the website say…

A price that you can afford. It’s free. No seat limits. No time-limited licenses. No worries!