Allot 2-3 per person if you plan to use them for beverages only. If you plan to also use them for the cake (as many couples prefer the printed beverage napkins than the larger ones), you must allot another 1 for every guest.
It's best to order a few more to be on the safe side. You don't want to run out of them during the reception. In case there are extra pieces after the event, the couple can always use it at home for their first few dinners as husband and wife.