The race to 'as much storage volume as possible' on that mechanical HDD is just brutal, announced a while ago but now starting to list are 18 Terabyte HDDs from WD, the gold series in specific. 

The 3.5-inch model WD181KRYZ is a unit to be found in the enterprise series WD Gold and seems to be priced in a 649 euros region. The spot of this HDD is interesting as WD does not even have a product page online just yet, but yeah it's already listed at etailers. In addition to the 18TB model, a 16TB model was released as well. The 18 TB version is listed as: 18TB GOLD 512 MB 3.5IN SATA 6GB/S 7200RPM

Before hitting that buy button, you might want to wait and see when the WD product page goes online, it is uncertain if the HDD will be CMR or SMR based.