Don't know the exact number, but I'd guess in the hundreds at least. Thousands of years of war, after all.

[ It could be less. It could be many, many more. ]

I knew how to use my shard at least a little, when I had one. Others know or have been taught, can use it even if they don't have a shard in their chest if they have one with them -- that's what Reynard did.