The supply of a stock in itself is meaningless, the only point of a stock is its USD value representation. In the case of a mass sell-off, sure, the same amount of stocks exist, but its monetary value sinks. Hence the market cap goes up in smoke.

This is also why a fraction of a market cap (say a few billion) can dramatically move a stock price of a 1 trillion market cap.

Same for Bitcoin. If its market price crashes, the same amount of Bitcoin exists. Yet this means nothing as only the BTCUSD pair is worth anything.