When you place a market order, the exchange will execute the order at the best bid/offer available. In the event that the amount of existing bids/offers is insufficient to match your order quantity, the remaining unexecuted quantity will be matched against the next best bid/offer.

Let's look at an example to help you understand:
If you place a market order to buy 3500 shares of ITC, 108 shares will be purchased at 263.20, 934 shares will be purchased at 263.30, and the remaining 2458 shares will be purchased at 263.35.
In a different sense, the same notion applies to limit orders. If you had put a limit order to buy 3500 shares at 263.20 in the example above, 108 shares would be purchased instantly, but the remaining 3392 shares would remain as a pending order in your order book, and the order would be executed if any offer at your price came in.

Note that if any of the bids/offers contain only the disclosed quantity, the quantity executed will differ from what is shown in the market depth.