Orders to buy or sell will execute based on the quantity displayed at the National Best Bid and Offer (NBBO) price at a given point in time. This is to ensure orderly executions and liquidity continue for stocks less than $1.
For example, if you place a market order to Buy 5,000 shares there are only 3,000 shares available at X National Best Offer price , then the system will execute 3,000 shares at X price and then continue to execute the remaining shares in similar fashion at the next available NBBO prices until the order is completely filled.
For example, if you place a limit order to Buy 5,000 shares of a security at $5.00 per share and the National Best Offer falls to $5.00 with a quantity of 3,000, your limit buy order will partially fill for the 3,000 shares. The remaining 2,000 shares will be filled similarly.