I have a Barbarian with Runescarred dedication. He is hasted and casts true strike and then makes a whirlwind strike. Whirlwind strike takes 3 actions but is essentially one strike (from feat name and narrative perspective), however, you need to "make a melee Strike against each enemy within your melee reach" when determining hit success. So, does the Barbarian roll twice to hit for all surrounding enemies, or roll twice to hit for first targeted enemy only?
Only the first.
Whirlwind attack mechanically is not one single attack but a bunch of attacks as the text clarify.
You attack all nearby adversaries. Make a melee Strike against each enemy within your melee reach. Each attack counts toward your multiple attack penalty, but do not increase your penalty until you have made all your attacks.
The name of the feat let assume that is one attack for all...and narratively speaking it is cool to think you are doing one big swing and try to hit them all but mechanically you roll once against every enemy and this means that you are making a strike (so an attack) against every enemy one at time.
So we are talking about attacks...and since we are talking about attacks true strike works only on the first
The next time you make an attack roll before the end of your turn, roll the attack twice and use the better result. [...]