Where does it say there are 16 shards?
Aona seems like a good fit for Sel. It seems like a good assumption to make.
To answer this question will take sifting carefully through WoK. So it's going to be a bit, and frankly will probably end up being done properly by somebody else, but since I made the claim, I thought I'd put out a basic defense of the idea.
Firstly, there are 16 Allomantic metals. 16 was Preservations signal to humanity, and it was chosen
because Ruin couldn't block it---it was too fundamental too the way the universe works. Sazed claims that there are implications of the number much deeper than he had fathomed even with his expanded mind.
With 16, everything comes in pairs of pairs of pairs of pairs. The symmetry is deep and very beautiful. Everything has its opposite, every pair has its opposite, etc.
Three of sixteen ruled---that's a pretty strong hint there. The only things that we have seen in the Cosmere capable of creating worlds have been shards. Three shards creating the world seems about right for the number of magic systems we are expecting to see, especially if Odium has had a hand in the world, and perhaps created a few more. (I suspect he has, and that we are going to see magic that makes Hemalurgy seem as innocent as collecting stamps.) And if Brandon said that three shards created Roshar---then there are almost certainly sixteen shards total. (Interestingly, the Almighty claimed to have created humanity. Perhaps the other two created other parts.)
The anonymous letter in the bumps in Part 2 refers to the "17th Shard," but by its very name it suggests that it isn't truly a Shard, but something else involved in the mythos that interacts with the 16 true shards.
In short, it would take a lot of counter-evidence to convince me that there are not 16 shards. It was already the most likely number anyway.
