Q1: I was wondering why it is necessary for San Francisco to even make such a small amount of coins every year. I mean when Philadelphia and Denver make billions of pennies per year, what is the point of San Fransico making just a few million of even a few hundred thousand. Q2: Who and what decides when the mintage for a coin has reached its end? Why aren't mintages rounded like 1.5 billion exactly instead of like 1,569,574,195? Is it depletion of materials, weakening demand?
As stated, SF produces Proof coins for collectors only. The mintage is for the most part determined by sales. The mint what they think they can sell at first, then as orders come in they mint more accordingly. At the end of sales, inevitably they will have some left over. These are then melted and the metal used in the years coins. The final mintage is ultimately determined by sales alone. As for your second question - mintages of business strike coins are for the most part determined by the Fed. Every year, the Fed tells the mint how many coins and of what denominations they will need the following year. The mint produces these coins as per those orders. They make no effort to stop mintage numbers on round numbers because it would require unnecessary effort to do so. They just mint up however many planchets they have in a hopper at any given time that comes closest to what the Fed ordered.
