Witchstalker Frenzy has the ability "This spell costs {1} less to cast for each creature that attacked this turn." It's well documented that this includes attackers that are no longer alive, or that are alive but no longer creatures; and that it doesn't include creatures that were put onto the battlefield attacking instead of being declared as attackers. Effectively, it just remembers the number of creatures that were declared as attackers during the Declare Attackers step. I believe the rule that leads to this behavior is 608.2i:
608.2i. Some effects look back in time and require information about previous game states and actions rather than considering the current game state. If such an effect requires information from the game about an object or group of objects, and that effect is not taking any actions on those objects, they don't need to be currently in the zone they were in at the time of that previous game state or action, nor do they need to currently meet the criteria described in the action, as long as they did so at the specified time. This is an exception to 608.2h.
Example: A player attacks with Bear Cub. Later in the turn, an effect causes Bear Cub to become a noncreature permanent. The same player then casts Search Party Captain, a spell that says in part "This spell costs {1} less to cast for each creature you attacked with this turn." That spell costs {1} less because the player attacked with a creature, even though the Bear Cub they attacked with is no longer a creature.
But what if there were two Declare Attackers steps, and the same creature attacked in both of them? For example, Fear of Missing Out might attack, untapping itself, and then attack again in the additional combat. It's not clear to me what 608.2i has to say about this, nor does Witchstalker Frenzy have a ruling clarifying it. It requires information about a group of objects, namely the group of objects that attacked this turn, and what it does with that group of objects is count them. Can an object (Fear of Missing Out) appear in that group twice, in order to be counted twice?
