This year, up to now, JP Morgan has hoarded as many as 20.4 million ounces of silver in its COMEX warehouse:
source: Simple Digressions
Interestingly, the JPM warehouse has been reporting higher silver stocks since 2015 (no matter at what price the silver was trading). If they continue hoarding silver at the same pace as in 1Q 2018, at the end of 2018 they would have had around 200 million ounces of silver (now there are 140 million ounces at JPM warehouse).
For better comparison, SLV, the world's largest private owner of silver bullion, holds 318 milion ounces of silver now (JPM Morgan also has its silver stake there).
Well, it is a lot of silver (global annual mine production is around 900 million ounces) so the question is:
Why are they doing it?