Does "whenever a creature attacks" count for each attacker separately?
Yes (though they will be lost one at a time).



There's a rule specifically covering this:


  603.2c An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 509.4.


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).