What happens when more than one creature dies causing a trigger
If an ability triggers each time a creature dies, then it triggers once for each creature that died, not just for each event where one or more creatures died.

Rule 700.1 says


  Anything that happens in a game is an event. Multiple events may take place during the resolution of a spell or ability. The text of triggered abilities and replacement effects defines the event they’re looking for. One "happening" may be treated as a single event by one ability and as multiple events by another.


in addition, rule 603.2c says


  An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences.


In this particular case, that means that even though the creatures died at the same time, they count as different events for the purpose of triggers that look for individual deaths.