trigger on an array of docs

Hi everyone,
I have some difficulties to find a parallelism solution for my problem . In fact i receive a soap request and i have to make N calls to a backend. I precise that N is the number of specific parameters defined in the request. So I have to make a backend call for each parameter.
the problem is that I have to define a trigger which joins all the “N” canonical responses (they are same). My question is: could we define a trigger that receives an array of documents? the size of the array will be contained in a technical canonical doc. So the trigger must receive the information number(“N”) in the technical doc and based on this information it waits to receive “N” canonical docs?

It difficult if “N” can be any number.
1 approach is to publish 1 document after you receive all responses but that means u will have to use some intermediate DB.

Are you using BPM or only IS ?