This question already has an answer here:
There are a lot of spells and even weapons the have multiple damage types in the same attack. For example, Meteor Swarm has 20d6 bludgeoning and 20d6 fire damage.
Let's say an unfortunate PC gets hit with meteor swarm and has 90 max hp, he first takes the bludgeoning damage of 91 and then the fire damage of 85. does this all register as 1 hit and become unconscious or would he become unconscious from the bludgeoning damage and then take the failed death saves from the fire damage?
In short: do multi element attacks count as 1 hit or does each element count as a different hit?