Card Text: Whenever a creature attacks you, its controller loses 1 life.
My question is: does the attacker lose one life per attacking creature?
Yes (though they will be lost one at a time).
There's a rule specifically covering this:
If your opponent attacks you with an Elf and a Bear, two instances of the event "a creature attacks you" occurred (the Elf attacked you, and the Bear attacked you), so the ability will trigger twice.
You will place these on the stack where they will resolve one at a time, each causing him to lose one life (for a total of two).
Yes, the attacker will lose one life per attacking creature.
The ability is a triggered ability, because it uses the word "whenever". The trigger is a creature having attacked, and if 5 creatures attack, then this will trigger 5 times.