Powervault capacity utilization for VHDX

I want to get the most utilization for backups out of my PowerVault. I have an old MD1000 that seems to burn up hard drives. For this reason, I will maintain 3 hot spares in the array. With the remaining 12 disks, is it better to have a single 12-disk RAID6, or two 6-disk RAID6? I/O will be bursty, but as this is a backup server, performance is not critical.

Additionally, how much of the virtual disk volume can I safely consume without creating an issue? If I have a 5TB virtual disk, mount it to a Windows server and create a 5TB volume, can I place a 5TB fixed VHDX on it without issue? Or do I need to leave 10% free space?