We have had occasions where we have been expecting 100 and found there to be 150 plus on the day. This not only creates a problem with regards to your invoice ! but also means that you run the risk of the bar running dry and service suffering. No-one likes to wait too long to be served and the situation can easily escalate into many unhappy guests. Bartenders are pretty good at head counts and you do not want a problem like this to interfere with your special occasion.