如何在IOS中通过AFNetworking在后台上传超过200张图像

I am trying to upload more then 200 images (as per my need) When application in background. I am using NSUrlSession (upload task). It is uploading approx. 20 images easily but after that process is not responding. I am using a single request (Not using an array for upload images on server for client requirement). Please suggest me any idea with some example code, Because i have tried with approx. 10-15 different approach. I am trying to upload more then 200 images (as