In most cases it should not be assumed that you will receive more than one key for your apartment.
If you do need another key, this would need to be requested from the accommodation agency and a deposit will more than likely be required before this will be handed over.
If a key is lost, depending on the resort, accommodation and agency, the cost to replace does differ.
Many apartments have specially cut keys that cannot be reproduced in resort and there is a hefty fee/charge to replace them. To avoid having to pay for a replacement please be vigilant with your accommodation keys at all times.