I am not aware of any limitation on the number of files. I think I have seen a demonstration using 7 images. Something to experiment with, I suppose.
I think I read that there is no advantage over using two files.
Really? I have used three images. And I have seen excellent HDR results posted here from others using seven images. I don't know where you would have gotten that information.
From Victoria Bampton:
You’ll get the best result with the fewest images – one for the highlights and one for the shadows. Lightroom’s method doesn’t need the images in between, and using more images increases the risk of ghosting.