The value of X is calculated only once, as Avenger of the Fallen’s mobilize ability resolves.

You choose the player, planeswalker, or battle each Warrior token is attacking. They don’t all have to attack the same one, and they don’t have to attack the same player, planeswalker, or battle as the creature with mobilize.

Although the Warrior tokens enter as attacking creatures, they were never declared as attacking creatures. Abilities that trigger whenever a creature attacks won’t trigger when the tokens enter attacking.