At the beginning of your upkeep, you may put a verse counter on Serra's Hymn.
Sacrifice Serra's Hymn: Prevent the next X damage that would be dealt this turn to any number of targets, divided as you choose, where X is the number of verse counters on Serra's Hymn.
You count the number of counters, then divide up the prevention during the announcement of the ability. Each targeted creature receives the appropriate amount of damage prevention when the ability resolves.