RSVP Etiquette for Hosts and Guests
Set a deadline three to four weeks before the event, be explicit about plus-ones on the invitation itself, and follow up once or twice rather than five times.
Plus-one policy
Decide the policy before invitations go out, show the reserved seat count on each invitation, and lock final plus-one decisions before vendor headcount deadlines.
Getting replies back faster
Use one direct request with a single clear action, tie the cutoff date to a real vendor deadline, and send two reminder waves rather than a steady trickle.
See also the event message templates and the planning timeline.