Generic request on postman

Hello I will use postman to test the REST API of a project And i search to make generic request for example :

I have a post request who make a research, the parameters of this research are in the body how can i automate the request with pre-request-script for modify body at each iteration.

Because I dont want to create a collection with 20 post request where we are just the body who change. I tried to use a while in pre-request-script or in test script but postman execute just one time the request. Then i tried to use the runner for call request 5 time for example and i writing a script who modify a variable each time, but i think that global and environment variable are reset at each iteration of the runner.

If you have idea.

Sorry for my english