First, know how many guests will attend the reception.
Most people make an error in estimating that number.
If 150 invitations are sent, there will be more than 150 people attending. There could be as many as 300. Double the invitation count, then take 65 percent and that will probably be the head count. It's a good idea to estimate that number before selecting a cake design, since the number of tiers in a cake affects the number of people it can serve. Consider the time of the reception.