If you would be dealt combat damage by multiple attacking creatures, you choose which of that damage to prevent. For example, if X is 3 and you’d be dealt combat damage by a 1/3 Scroll Thief and a 3/5 Siege Mastodon, you might choose to prevent 1 damage from the Scroll Thief and 2 damage from the Siege Mastodon. You don’t decide until the point at which the creatures would deal their damage.