Can "Produce a Miracle for the Masses" be used to replace multiple cubes?
You can only do it once.

When you see a cube on a card, it means one cube. This isn't explicitly stated in the rules, but it should be apparent from the way cards always show a number of cubes, and any time there's not a known number of cubes, it's qualified. For example, this very card says "...any {white}..." rather than just "...{white}..." to make it clear that it triggers just once when you receive any number of clerics. If you look through other cards, I think you'll find you've always interpreted a single cube to mean a single adventurer.

So you could more clearly transcribe the card as:


  Once per round, whenever you take an action that provides any clerics, you can also replace a fighter, a rogue, or a wizard with a cleric.


And since it says you can only make that replacement once, you can only replace one cube.