I currently have a Synology NAS system with two 4TB hard drives. In RAID-1, this gives me 4TB of storage. This is not quite enough for my desktop (2TB) and notebook (1TB). Professionals recommend that you do 4 times the combined storage of all computers. In my case, this will be a minimum of 12TB. However, one can never have too much storage. I will rather acquire two 18TB drives which will give me a total of 18TB in RAID-1. If one drive fails, the other drive takes over, and I will need to replace the failed drive.
The two hard drives will be pricey. Each drive is $730 which comes to $1460. This is expensive, but my music collection alone is worth more than the two drives. In addition, there will be hours if not days worth of ranking the music over again.