There must be an algorithm we can use to determine the number of votes given the number of submissions and authors.
Less authors = more votes needed
More stories = more votes needed
Again, we are evolving as we go.
I feel simpler is better. Regardless of the number of votes available, I find that I always have a subjective, almost arbitrary, cut off line. Having dynamic voting numbers isn't going to change that. Let's just keep it simple and fun for everyone.
If a writer is inspired and wants to submit multiple stories, then great. Isn't inspiring writers the main purpose of these challenges? As a reader, I find it enjoyable to take an hour or two and read through the entries in one go. It's a nice break from other things and it's interesting to see different approaches to a common prompt. I also enjoy looking at the voting results and comparing my votes and short list to the voting by others. I usually find my tastes seem to be outliers, which is sort of cool.
I do not believe that using an algorithm to determine number of votes would enhance any of this. So just go with the simplest thing that could work.