Like said before, get it all in writing. When you get the head count of people eating, figure out how much food you'll need and then add some more to cover last minute add-ons, overeaters, mistakes, dropped food, etc. Are they expecting you to serve, too? Side dishes? Or...just meat.
Once you get the facts, make a plan, including a time table and stick to it. Many here will help you get through it.
Blessings on your preparations and future cook.
IMBAS Certified MOINK Ball maker and IMBAS Sgt. At Arms.
And now, brethren, I commend you to God, and to the word of his grace, which is able to build you up, and to give you an inheritance among all them which are sanctified. [Acts 20:32 KJV]