You seem to be coming at your problem from the wrong end. When you first open Publisher you can choose 'Business Cards' as the type of document you want to design. Clicking through the various options should not only show the size of the card but also the layout on the page (e.g. 2 columns of 5 cards). When you then go on to design your card you'll only see a single card on the screen, but selecting 'Print' will automatically print a full page of cards (rather than a single card).
Chris