Hi Cheeus, I live in Spain and it is normal here that the number of your National Identity Document (which all Spaniards and foreigners resident in Spain are issued with and are required to carry on your person) is requested and used for any and every sort of transaction, commercial or bureaucratic. In a country of 46 million people, with many people sharing the same or similar names (imagine how many John Murphys you would have in Ireland if there were 46 million of us!) it is essentially your personal identifier. For non-resident foreigners therefore, a Spanish business would normally automatically ask for a passport number. Unless there is some other reason that you think the apartment rental company is dodgy, it sounds a pretty normal request from a Spanish perspective.