Question Around How Coin Mintages Are Determined

The presses have counters on them.

Of course the count isn't accurate and some of the things struck by the presses aren't really coins but they arte good approximations. I'm sure they also compare these numbers to shipments of coins which is probably a little more accurate.

In early January each year they change dies to the new date and start over.

In the old days they counted coins that went into shipments but now days they weigh them.

What I wonder is how collectors determine how many coins of a certain type still exist 90 years later such as many of the Standing Liberty quarters? We know that the dates of the pre-1925 coins wore off quickly and we know how many were minted. How do we know how many still exist since they have been sold as junk silver when dates are gone? Every time the price of silver hits a new high silver coins are melted down because they are in such bad shape they have little numismatic value. Same goes for the Mercury dimes and LWH dollars. We know original mintage but where are the coins now? I see that the Fed or some or the banks did not even ask for silver coins during certain years during the Great Depression. Now it seems all the circulating coins other than half dollars and dollars are minted in vast quantities. There are certain coins that are not produced in vast quantities such as 1973S and 74S silver clad Ike dollar. The 1999P, Proof of the Susan B. has mintage of only 750,000. The Sacagawea dollars have a few years with pretty low mintage in 2002, 2007, 2008 but even in proof condition sell for ten bucks. The mint produced 767,140,000 in 2000 from Philadelphia alone. After 2001 all the mints stopped producing vast quantities. Every time I think I have it I always find exceptions to the rule. I can contradict myself in the same post.