If you have not contracted the services of a Wedding Coordinator, before signing any contract (and specifically that for hotel accommodations and your venue), scan for a little term called " Guarantee". This one word can make all the difference between enjoying the planning of your big day and being completely stressed and unprepared .
You know the term "ignorance is bliss"? Well it applies here as well. In the case of a Venue, the guaranteed amount referenced on your contract, is the amount that the signatory is guaranteeing the Venue for food and beverage costs. In order for the Venue to "rent" you their event space, they must be assured a minimum amount of revenue on their room. On your contract, it may be reflected as a headcount, i.e., 90 people, or no less than 10% of the headcount stated, but it all equates to the same amount. Before signing your contract, ask the Sales Manager to confirm the guaranteed revenue amount in terms of dollars. Take into account that this amount does not include taxes. This is the number that regardless of how many of your guest's rsvp they will be attending, you are responsible to your Venue for.
In the event, your headcount falls below the minimum you have stated, do not panic. Speak with the sales Manager regarding options for upgrading your current menu, bar, etc. Although you will not feed as many guests as you originally intended, you will be able to upgrade your options for those that are able to share in your special day.