If you are going to get server 2016 it would be worth having a read up about ReFS integration with Veeam as well https://www.veeam.com/blog/advanced-refs-integration-coming-veeam-availability-suite.html
If you know what your daily change rate is you can use the restore point simulator that was already posted to see if 20TB is enough in terms of capacity, it might not be. Also take a look at the Veeam best practice guide https://bp.veeam.expert/
to make sure the server is appropriately sized in terms of memory and CPU etc.
Also have a look through the forums for people who have had some issues with windows dedupe in particular when backing up large VMs.
Preparation is key