It's entirely up to you Brian but I have been looking at My Cloud EX2100 12TB but if you mean an empty box and need hard-drives to fill them, I use WD 3TB and my backup solution is very simple, in that I have two caddies, so just slot them in, and away I go.
Are you looking to work as RAID 0 (all drives used to simulate a single large disk), RAID 1 ( 2 pairs of drives mirrored) or RAiD 5 (4 drives sharing the data with redundancy = approx 3/4 the total combined disk capacities) or JBOD (Just a Bunch Of Disks i.e. 4 individually named drives)?
I use a Synology box with 4 x 4tb drives so 16tb in total. About 10tb is used. No redundancy set up as want so also have another backup of everything on smaller 1tb - 4tb external drives. Also have a Time Capsule that does constant backups. Depends how important the data is to you.
I would use a 'WD red NAS 27/7'4TB drive. If you just use the NAS box as storage and they show up as independent drives then back up each drive with external USB drives [belt and braces approach] If you do this then there is no point in setting up RAID and loosing space [4tb on RAID5]. Your backup drives do not have to be expensive ones as they are not being consistently on.
Seagate now have 10GB drives on the market in case anyone's wondering, not cheap though but they're there if your needs require them.
I'd personally estimate your data needs for (say) the next three years (which is a decent enough approximation for the lifecycle of working/regular use drives and you'll likely be replacing them after such time). So if you shoot 2TB per year and have no immediate desire to change cameras (which will likely be higher pixel count) then you'll need at least 6TB spare.
Bear in mind that it's also a good idea to have mirrored (or partity) storage with NAS boxes so the array can be reconstructed in the event of a drive failure. A secondary backup (eg. external drive) is also recommended.
Another thing worth mentioning... do double check on the NAS manufacturer's website for drive compatibility. When I bought mine there were certain NAS drives that were incompatible with the NAS box.
They also seem to publish a lot of firmware updates which improve drive compatibility and reliability, and as such it's the first thing that should be done (IMO) prior to formatting the drives and uploading data to them.