Ureni, the Song Unending

Flying, protection from white and from black
When Ureni enters, it deals X damage divided as you choose among any number of target creatures and/or planeswalkers your opponents control, where X is the number of lands you control.

Tarkir: Dragonstorm Promos

Tarkir: Dragonstorm

You cannot choose more than X targets for Ureni’s triggered ability. You choose how the damage is divided immediately after choosing those targets. The value of X won’t change even if the number of lands you control changes after that point. That division of damage also won’t change, even if damage can’t be dealt to one or more of those targets as the ability resolves (usually because they’ve left the battlefield).