A lot of dances can be done with a flexible number of couples. Read your cards and put a note at the bottom.

At some wedding dances I call the Hokey Cokey, or Horses Brawl (my simplified version).

For the first dance, one option is for the band to play a waltz (or put the couples favourite CD on...
Call the Bride and Groom onto the floor,
after a little while call the bridesmaids, best man etc to bring partners onto the floor,
then the bride and grooms parents
then everyone.

If you join Mudcat we can send you a pm with other dance instructions, e.g. a wedding dance with a story.