Does Veeam B&R have limitations on the type of Disks used for creating a Backup repo?

RAID 6 across 12 20TB disks is actually quite a good configuration IMO.  I tend to use RAID 5 for better disk efficiency but this does create more risk as it only allows for one parity disk whereas RAID 6 would have two parity disks, though the write penalty is higher at 6 vs 4 with RAID 5.  However, in most cases with writing backup data, I don’t believe that it will terribly matter in most cases, especially considering how many spindles you’re writing across anyway.  RAID 10 is probably acceptable as well though you give up usable space for performance.  Like with most disk configurations, it’s really going to be up to what fits your application the best.  In the case of a backup repo, RAID 5 or RAID 6 is completely acceptable.

As Chris mentioned, since you’re using Windows, it would be recommended to format the volume using the REFS filesystem using 64k blocks to take advantage of block cloning and the massive space savings that goes along with it.  If you were using Linux, then the recommendation would be to format using the XFS filesystem for the same reason.  I’ll note that I only recommend REFS when using a battery-backed cache on the RAID card.  Most standalone servers this is typical, but if it were something like a large Synology NAS or similar that uses a software RAID, I’d look at another filesystem such as NTFS just because I’m not really trusting of REFS without the cache in the event of a power failure, etc.  REFS has gotten much better over the the years, but both myself and a former colleague (now client) have both run into issues with REFS corruption due to hardware/power failures.