Looks like some white level issue; if you have the input DNGs, I can take a look.
Don't remember about any limit on number of files, but I'm not sure why you need more than 10 images. This is not a general image stacking algorithm - it only does a dumb copying of the brightest pixels from each image. There are a lot of stacking programs, widely used in astrophotography.
Just found another program that seems to use the same algorithm:
http://jcelaya.github.io/hdrmerge/I didn't try it, but I should probably stop reinventing the wheel
