Mike Mearls is not an official judge of the rules as written; he's the co-creator of 5e and the creative director of the franchise, but Jeremy Crawford is the lead rules designer and managing editor for D&D (and the lead designer of the PHB). And according to Crawford:
Do you roll concentration for every instance of damage taken? id est every Magic Missile hit?
Concentration: "You make a separate saving throw for each source of damage" (PH, 203). Roll for each missile.
Mearls comments on the discrepancy between their responses here:
Regarding magic missile Jeremy Crawford Roll to keep concentration for each hit. You say for purposes of dmg/death it’s 1 hit.
Jeremy tends to use existing rules and extrapolate from there. I tend to build on my programmatic understanding of underlying design intent. [...] That's why he's a better source for how the game works as published - I'm better versed in the concept/structure than the final execution.
In any case, the rule Jeremy Crawford cites makes it clear what the RAW answer is:
Any spell that causes multiple sources of damage (e.g. spells that shoot multiple rays at enemies - if the caster shoots them at the same enemy) would cause multiple concentration checks.