I attach a spreadsheet which has been handed down to me from various brides who assure me it is really helpful for before you get your invites or date saving stationery out, and more importantly the RSVPs back.
Basically you fill out your guest info as normal and after each couple/family you put the percentage chance they will attend. The spreadsheet then adds it all up and churns out an estimated number of guests at your wedding based on all your percentages. It's pretty rudimentary but is helpful for estimating costs and accommodation, OOT bags etc.
My sister said it was pretty accurate, but you have to be brutally honest about whether people will/can attend, also helps you factor in people who you have to invite but certainly won't come due to cost/distance etc. It's a planning document probably for your eyes only as feelings may be hurt!!
Hope it is helpful to someone,