We invited just over 250, since we both have big families (thats about 200 of them). 9 Months out and 88 have booked. We budgeted for 100 max.
Before we sent them out we had a good idea of who would forsure come, and for 'maybes' we counted them as 1/2 a person, and then we added 20% for people who could surprise us. So far it worked out to be pretty accurate. The hard part was going to the travel agent to look for different costs and saying ' we will likely have between 40 and 100 guests' hehe.
Turns out most tour companies from my city can only take 60 guests, so we've had to work with 2 companies :-(