Just one, for the device to which you are restoring the data - backing up is a free feature!

According to Apple an iPhone's average ownership is approximately 3 years. So someone could decide to purchase a single Device License for that time, and as they upgrade to a new iPhone simply top up with another device slot. That same user, knowing that they are considering replacing their current device in a near or certain future could alternatively cater for that, and purchase 2 Devices License from the get go.

It's actually very flexible, and a fair way of ensuring that the we, the software developers are getting paid for constantly maintaining the software and ensuring it works as intended in an ever changing Apple framework environment, while constantly adding features - besides FWIW: we haven't charged our users for a single version update since the release of iMazing 2 in 2016 thanks to this model.

PS: We understand misfortune can strike and are certainly not interested in profiting from such an event. If your Apple device is lost or broken, please contact our support team for assistance with your license.

Hey sorry you don't like our licensing model, but to be fair we do offer a device-based license model as an alternative to the now ubiquitous yearly renewal license model (our Unlimited Device License).
The latter works best for users that want flexibility and need to use iMazing with various and new devices on a regular basis, typically when helping out their entourage managing their devices.
The former, Device License is more casual or entry level alternative that offers full features guaranteed to be working and updated with every release, during the entire life of their device ownership.
Hope this sheds a different light on the matter ;-)