Arcana Domain clerics gain the Spell Breaker feature at 6th level. The description (SCAG, p. 126) says:
Starting at 6th level, when you restore hit points to an ally with a spell of 1st level or higher, you can also end one spell of your choice on that creature. The level of the spell you end must be equal to or lower than the level of the spell slot you use to cast the healing spell.
The description uses the phrase "an ally" which would generally indicate that the Cleric heals one person and can also end one spell on them. But there are a number of spells that can heal en masse; Mass Healing Word for instance.
Since the heal can affect multiple allies at once, can the Cleric choose to remove one spell from each of them, or is it limited to only one ally even if healing more than one?
Best Answer
Yes, this works en masse
When you cast a spell such as Mass Healing Word, every ally healed by it is "an ally" for whom "you restore hit points to", making each of them a viable target for your Spell Breaker feature. Notice that the feature gives no restrictions on how often this may be done. This allows you to do this any number of times in a given timeframe, or even for several targets at once, as long as each target meets the criteria.