A level 20 barbarian has the Primal Champion class benefit, which allows him to have a Constitution score of 24 (and a modifier of +7). The Durable feat gives your Hit Die a minimum equal to your CON modifier ×2. But the barbarian hit die is a d12.
Do you regain 12 or 14 hit points, since your maximum is 12, and your minimum is 14?
Best Answer
You Regain 21 Hit Points
That at least seems to be the final word from Mike Mearls: http://www.sageadvice.eu/2014/09/01/durable-feat/
He asserts that a character can use durable to get back more than max possible hit points. For your barbarian, when you roll the hit die, the result is 14 no matter what you roll, and then you add your Con modifier to it.