Always plan for 100% attendance, and any money you save at the end is a bonus! Most vendors don't require final counts until 3 or 7 days before. We had 92% acceptances for our destination wedding, despite every single attendee needing to fly to get there.
Agreed with the pp, always count on everyone you invite. We had the first snow of the year the night of DD wedding and only 4 guests didn't show and 4 guests brought friends or dates that they didn't rsvp. It worked out but glad we were prepared.
Re: Guest List Count
Always plan for all of your guests coming.