From County Bride's Etiquette Pages:
The Reception - Seating Plan
To avoid confusion it is essential to have a seating plan. Handwritten
place cards - in the style of the invitation, numbers on tables and a
plan of the room will help guests find their seat with the minimum of
fuss. The ushers can be utilised to direct arriving guests to their
table and help any who are having difficulty finding their seat. It may
be a good idea to have a few extra places available for unexpected guests.
See more information here