6dc5455c____The_Coppermind___17th_Shard__Entry
[Chapter] 72 [Entry] 
Yes, there are sixteen metals. I find it highly unlikely that the Lord Ruler did not know of them all. Indeed, the fact that he spoke of several on the plates in the storage caches meant that he knew at least of those. I must assume that he did not tell mankind of them earlier for a reason. Perhaps he held them back to give him a secret edge, much as he kept back the single nugget of Preservation's body that made men into Mistborn. Or, perhaps he simply decided that mankind had enough power in the ten metals they already understood. Some things we shall never know. Part of me still finds what he did regrettable. During the thousand-year reign of the Lord Ruler, how many people were born, Snapped, lived, and died never knowing that they were Mistings, simply because their metals were unknown? Of course, this did give us a slight advantage, at the end. Ruin had a lot of trouble giving duralumin to his Inquisitors, since they'd need an Allomancer who could burn it to kill before they could use it. And, since none of the duralumin Mistings in the world knew about their power, they didn't burn it and reveal themselves to Ruin. That left most Inquisitors without the power of duralumin, save in a few important cases—such as Marsh—where they got it from a Mistborn. This was usually considered a waste, for if one killed a Mistborn with Hemalurgy, one could draw out only one of their sixteen powers and lost the rest. Ruin considered it much better to try to subvert them and gain access to all of their power.
The Hero of Ages/Epigraphs - The Coppermind - 17th Shard
Entry