An increase in the money supply, then, only dilutes the effectiveness of each gold ounce; on the other hand, a fall in the supply of money raises the power of each gold ounce to do its work.

We come to the startling truth that it doesn’t matter what the supply of money is. Any supply will do as well as any other supply. The free market will simply adjust by changing the purchasing-power, or effectiveness, of its gold unit. There is no need whatever for any planned increase in the money supply, for the supply to rise to offset any condition, or to follow any artificial criteria. More money does not supply more capital, is not more productive, does not permit “economic growth.”

What Has Government Done to Our Money, Murray Rothbard; pg 33-34

The “or to follow any artificial criteria” in the quote means: Rothbard does not actually prefer a fixed money supply. He prefers a money supply that is determined by the free market (specifically, by voluntary market processes under a commodity standard like gold), not one that is deliberately fixed at some arbitrary number by anyone — whether a government, a central bank, or even by some supposed “ideal rule” (eg: Bitcoin protocol). Source