Seagate X18 - 18TB Drive may be coming soon
WD and seagate have both been competing to bring the large capacities of Enterprise Nearline (ENL HDDs) to the market. ENL HDDs of 12TB are the biggest contributors to the hard drive revenues where margins have not eroded away like the lower than 6TB capacities and its understandable that all the manufacturers are tying to push the limits on the high capacities.
The lead Seagate had with its early introduction of X16 - 16TB Exos , was lost in July when WD introduced its 18TB Ultrastar. Seagate has been developing the 18TB though, and rumors are that in September we may see the New Exos X18 - 18TB drives. ( ST18000NM001G ?)
Elsewhere - Toshiba has 16TB as the highest capacity for now with its MG08 Platform.