Well, the tang-police not withstanding...
It really does "depend". 2 similar tangs - like a powder brown and a powder blue - will probably need several thousand gallons to be in the same tank. Yellows get along fine - I have 3 in my 215. Blue hippos will also do well together. You should also add the most aggressive one last. That can work, but my Sohol still took out my powder blue and my orange shoulder tangs - both of which I'd had for quite a while.
So, I'd suggest that you put together your list, and then start adding them. At some point in time, you may very well have a problem - and once you lose a tang, it's really, really hard to add it back - the aggressive ones will take it out.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."