about 5 years ago we bought a 2 TB server (big for 5 years ago) thinking we would be all set. However we kept filling drives, so we wound up having closer to 4 TB, meaning only half was online at any time.
As it turned out, we did not actually need instant access to most of what we were archiving, so we switched to swappable SATA external drives, which we now have close to 100 of in various desk drawers.
So now each man is an island. Each person keeps track of what is on what drive, using either an Excel document or lined paper. One has to be good about keeping these lists updated.
For assets we use more often, like graphics, music files and stock photos, we keep these in redundant locations around the network.
I guess it depends upon your business – for broadcast work you probably want more available online, same for episodic work.
Mike