You count the number of Clerics you control at the time the damage would be dealt.

If the same source would deal damage to a player at different times, you may prevent X of that damage each time.

If a source would deal damage to multiple players at the same time, you may prevent X damage that source would deal to each of those players. The choice for each player is made separately.

The player who would be dealt damage chooses the order in which to apply prevention and replacement effects that would affect that damage. This includes effects that involve choices, like the prevention effect from Battletide Alchemist's ability and the redirection effect that's part of the planeswalker rules. Those choices (such as whether you'll actually use Battletide Alchemist's ability to prevent that damage) are made as the relevant effects are applied.