The larger the tank, the better...even for one. If you want more...that changes things. These are big fast tangs that will cover 5-6' real fast. If you've seen them in several thousand gallon systems, IMO, you will see them at their best as far as a captive system goes.
The more I see these fish in particular in large systems, the more I really wonder about keeping them in smaller (even 300g) systems. But as far as home tanks go, they would be at their best, IMO, over 200g