doing some math
Just like the Anonymus Coward pointed out, it's 15 x 52 TB RAW for the Flash Array per 4U Enclosure.
These 780 TB RAW can be de-duplicated to get even more out of them. Let's take a very modest 3:1 factor putting us at 2.3 PB usable capacity.
For the NeatApp this would result in 1300 1.8 TB disks, which sounds reasonable, but I haven't factored
in the 30% RAID loss yet. But in this case we are talking either about a fairly new array of 2TB+ 2.5" SAS disks or 3.5" SATA disks.