Encouraging attendees RSVP will give you some idea of how many people to expect. Actual attendance will vary depending on your community and outreach and publicity efforts, as well as variables like the weather and any events that might be happening on the same day. While it's ideal to have a good crowd, don't be discouraged if fewer attend than you'd hoped; an event with modest attendance is still meaningful to the people there. If you can, set up the venue so it'll feel cozy if attendance is light, but make sure you have plenty of chairs available in case you need to add more seating.