DSCS9 is one of the newest recommended devices from DSDevices indeed.
DSDevices will be able to provide you with Xibo for Android licences as well as the hardware itself, and yes each device will need a licence - Licencing your Xibo for Android Display
Regarding monitors connected to the DSCS9 to show the content, there is no list of recommended ones, it highly depends on what you require, the most common solutions should be some 1080p screens with the size that will suit your project the best.
You can and most likely want to have all those displays connected to one CMS, as it will be easier to manage this way.
CMS Installation notes are here - https://xibo.org.uk/manual/en/install_cms.html
If you’re able to use docker for this, then that would be the recommended way, but you can use different server if you prefer.
On the android box itself, you only need to install Xibo for Android and connect it to your CMS - actually DSCS9 come with pre-installed Xibo player already - Installation Guide
If you’d have further questions, please do let us know.