dnd 5e – Does Alarm caster know if alarm has been triggered by multiple creatures at the same time?

It won’t work, you’ll be detected

The appropriate part of the Alarm spell says:

Until the spell ends, an alarm alerts you whenever a tiny or larger creature touches or enters the warded area.

Nothing in the spell mentions anything to do with timing. Two creatures have entered and so the alarm spell alerts the caster twice.

You might be thinking that the two sounds would overlap, hiding one alarm under the other but the simple rule is: Spells do what they say they do and this spell alerts the caster whenever a creature enters the zone.