Arcane Concentration lies to you: it says every time a spell damages an opponent. It only worked this way for the Beta when people realized it was retardedly OP for grinding. Perhaps it was reset this way by accident or through a glitch more recently, but it has been fixed and it was unbeknownst to me.
It now goes off on the initial cast (with a tiny delay, at the same time the mana is taken) once and that is it.
There are certain techs and items, however, that DO have chances/effects on each hit or each critical strike of AM.
And yes, Rank 1 is 3 missile launches, Rank 2 is 4 missile launches, and Rank 3 is 5 missile launches. After that they are all 5 missile launches. The amount of launches does not play into Arcane Concentration, however.
To answer your next question, on AEs such as Arcane Explosion and Flamestrike, the chance to AC comes as the spell hits the target but it is only x1 chance no matter how many you hit. Blizzard works similarly to Arcane Missiles. All projectile spells go into effect when the ball reaches their targets.
For the record, Arcane Concetration is still a very good spell.