File -> Print Setup
and there change the page size to the size of a business card (use a custom one)
perhaps with some extra space, depending on who will be printing them, some companies ask you to add a little border; make the card a bit larger then required, to prevent white borders....
then make a business card in that page.. and export as a PDF..
if you want to make multiple cards on one page, just choose the Shape -> Rectangle, and then use the inspector -> metrics and change the size to the size of the business card (maybe a bit bigger...).. and duplicate that rectangle until you have enough on that page..