When the Mac Pro was released on December 10, Apple promised that an 8TB SSD optionwould be available in the near future, and as of today, it's possible to choose the 8TB upgrade option when customizing a Mac Pro on Apple's site.
The 8TB SSD upgrade option is priced at $2,600 when adding it to the base Mac Pro model, which ships with a 256GB SSD. The previous top tier SSD was a 4TB upgrade that cost $1,400.
It's not entirely clear why Apple needed to delay the 8TB SSD storage option for the Mac Pro as it became available just a little over a week after the Mac Pro launched.
Apple has also promised that Radeon Pro W5700X and dual Radeon Pro W5700X GPU options are also coming soon, but those did not get released along with the 8TB SSD upgrade option.
Source: Macrumors