You may pay {W}{U}{B}{R}{G} rather than pay this spell's mana cost. If you do, X is 5.
Bringer of Green Zenith's Twilight enters the battlefield with X +1/+1 counters on it. If X was 5 or greater, it enters with twice that many counters instead.
When Bringer of Green Zenith's Twilight dies, shuffle it into its owner's library.