When you place a market order it gets executed at the best available counter orders on the exchange. If there's not enough quantity in the existing bids/offers to fulfill your order, the remaining unexecuted quantity will be matched against the next best bid/offer.

Example Scenario - Let's say you place a market order to buy 3500 shares of ITC:

1. 108 shares will be purchased at a price of 263.20.
2. 934 shares will be purchased at a price of 263.30.
3. The remaining 2458 shares will be purchased at a price of 263.35.
4. The same concept applies to limit orders in a different way. If you place a limit order to buy the 3500 shares at 263.20, 108 shares will be bought immediately. However, the remaining 3392 shares will remain as a pending order in the order book. If any offer at the specified price comes in, the order will be executed.

Did you know? If any of the bids/offers have a disclosed quantity, the quantity executed may differ from what is seen in the market depth.