First off, I'm not sure if this is where Ideas/Theories are to go. So I'm sorry if this is not in the right place.
I'm not sure if this idea/theory already has been discussed (as I have not looked too far back on this forum), or if Brandon has said anything about this, but as I was re-reading Oathbringer I had a thought as to why there might be specifically 10 books planned.
So, I know that the series is going to be split up between the first five and the last five books and that the second five in the series are going to act as a 'sequel' to the first.
My idea is that the ten individuals that we learn about in each of the books (the people who the books focus on) are going to become the next set of ten Heralds that get trapped to try to hold Odium and the Fused Spren on Braize (Damnation). Let me know what your thoughts are or if there is a glaring problem with this idea. I just wanted to see some other opinions.