Does that essentially rule out any spell attack from ever invoking more than one concentration check? (i.e. What about damage over time?) Are there any spells that can invoke multiple concentration checks - particularly in a single round? (To further clarify - saves are invoked for each source of damage - thus are there any spells that can cause multiple sources of damage - or will a spell always be just a single source of damage?)
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.
Multiple beams from a Warlock's Eldritch Blast should work. It's one cantrip, but above 5th level it has multiple beams, each of which is rolled to hit separately.
Firebolt, for example, is one beam, that has one roll to hit and the damage goes up with level, while Eldritch Blast is specifically called out as separate beams each with its own roll to hit. So Firebolt would be one concentration check, although perhaps a high one due to all the damage at once, whereas Eldritch Blast would be a lower concentration check, but a check for each bolt that hit.