The current maximum number of files (read: images, audio or video files) that can be uploaded with a story is 10, even if all files have the same single source file on the client's machine. Consider increasing this to a larger number like 30 or 50.

The relevant line in source can be found here:
https://github.com/fluid-project/sjrk-story-telling/blob/a870c8afe9609a484f9005778039a0fe73974545/src/server/middleware/saveStoryWithBinaries.js#L22