The number of stocks bought is always equal to the number of stocks bought - stocks crash because the price the marginal buyer is willing to pay goes down. Some

The number of stocks bought is always equal to the number of stocks bought - stocks crash because the price the marginal buyer is willing to pay goes down. Sometimes this is because there are too many sellers and the buyers can take their pick, but it need not be; it can also be because of a change in what buyers perceive the value of the stock to be.