Or consider this approach - no discount for 'missing' friend (s) - by doing this: offer a 'Base price X for 2' + Z per extra guest, but encourage/explain to the applicant to ~only~ book for 2 until they are sure of their ultimate number. Since I am booked so far in advance, I adopted this policy and never has a discount for a missing guest come up in the first place, even when it happens. It also takes the pressure off the person that initiated the booking.
Of course, if it is absolutely no cost to you if less people show up than expected, then the discount is a good-will gesture on your part.