The rules for Dungeons & Dragons are complex. They stretch over several books, and are sometimes open to interpretation. Wizards of the Coast have a special Sage Advice webpage to answer rules questions, and sites like rpg.stackexchange.com discuss rules at length. Now normally I try to stick to these expert opinions on how the rules should be read. But recently I came across a case where I didn't like the official answer. The question was how many concentration checks a spellcaster would need to make when he got hit by magic missiles. The official answer is one check "per damage source" and while one developer interprets magic missile as being one source, later another dev said it would be one roll per magic missile.

Rules-lawyering aside, I decided that when I am DM, a magic missile causes you to roll only one concentration check. For me that is not a question of rules, but of game design. How easy do you want to make it to break a spellcasters concentration? Even a 1st level wizard can cast 3 magic missiles, and they always hit, with no attack roll required, or saving throw possible. Even if the target had for example a 70% chance to succeed a DC10 concentration check, three checks drop that chance to 34%. Unless you basically cheat as a DM and give all enemy spellcaster the Shield spell, the game design question basically becomes whether you want NPC spellcasters to use concentration spells at all. And I certainly do, because a Wall of Fire cast across a battlefield changes the tactical situation and is thus more interesting than a Fireball, which is over instantly. D&D has a lot of interesting concentration-based spells like that, and you can build more interesting encounters if you use them and don't make them trivial to dispel.

Oh, and if you have a rules lawyer player who doesn't agree, tell him that if a Magic Missile is 3 separate damage sources, an NPC spellcaster casting Magic Missile on an unconscious player character will cause 3 death saving throw failures, and thus kill the player instantly. Most players will not want that. :)