All price changes in the stock or commodities markets impact the money supply to the extent it results in a change in margin credit. Do note that the amount of credit itself must change and not the price alone. A crash only has an impact if it results in net changes in the amount of credit.
This is a second order effect of a crash and not a first order effect. It also usually has no material impact on the money supply. Although a crash may have a large impact on notional concepts of wealth, the effect on the supply of money itself is usually neutralized by the banking system. It is normal, even in the 1929 crash, to see bankers announce they will keep open the brokerages' lines of credit and stand ready to purchase safe assets such as Treasuries in unlimited quantities from these brokers to hold the money supply roughly constant. If brokers sold enough treasuries, it could even expand the money supply.
The size of the money stock (M2) is 13.4 trillion dollars. The total size of all margin credit from all sources is 554 billion dollars. Although this is four percent of the money supply, which is a lot, you are also forgetting that in a crash margin purchases will begin happening too. Just as brokerages will be issuing margin calls, other investors smelling blood in the water will be purchasing on margin, though this will be a smaller effect.
The Fed normally drops the interest rate to expand the money supply after a crash to compensate for the loss of money created by the reduction in margin credit. Remember that 554 billion would not be wiped out in a crash, it would just be reduced, at least initially, as brokers required repayment.
