5TB is a pretty good size. It depends how you’ve got files distributed and how you collect so much data.
Previously we used Intralink for all of our ultra heavy + versioned CAD data, so that lived on it’s own server. Our ID department only used about 2TB over the course of ~6-7 years BUT that was for master copies, not development work that was stored on local backups. .5TB per designer is pretty reasonable - BUT keep in mind the costs associated with keeping trash data. You probably don’t need all of an interns versioned saves for something (or you may when you find out Rev C had some important data in it). The more junk data you store the harder of a time you’ll have maintaining it and indexing it for searching in the long run. That comes down to good process - storing final versions of files in a consistent folder structure for example, then only retaining WIP projects for 1-2 years for example.
Data storage is pretty cheap these days all things considered. Check out some of the WD Cloud or Drobo stuff. You’ll want to make sure you’re running a RAID V configuration or similar to minimize the fallout if 2 disks crap the bed.