Each guest you invite will eat approximately 1 to 1 ½ lbs. of food--approximately ½ lb. of meat per person, ¼ lb. of starches such as bread or pasta and ¼ lb. of vegetables such as salad. For a party of 60 this will be approximately 27.2 to 40.8kg. of food.
It Depends on Several Factors
The type of party you're throwing plays a big factor into how much food you will serve. A party of 60 children will eat much less than a party of 60 Marines. Other factors that you should consider are the time of day the party will be held and the focus of the party--whether the party about food, or food is only a formality.
Most experts agree that for a party of 60 guests you should provide each guest with at least 0.454kg. of food. This is a good rule of thumb as some guests will only nibble as others will take much more than 0.454kg. You should also take beverages into account--approximately two beverages per person, or 120 beverages.