I think a lot matters on what your definition of catering is here. Are you actually supplying full service of all food & drink plus service and all needed items…plates, flatware, napkins? Or, are you just talking about selling your products in bulk form for family or gathering type events with the food being your only offering?
A lot of restaurants around here offer bulk / buffet style options for their products. They offer discounts for large orders but other than that it is sold on a piece by piece basis. Which direction are you looking to go? Also, I wouldn’t really limit yourself to a base number…as I have had small business meetings catered with less than 10-20 people that a full meal service was provided on site and I am sure the supplier made out ok in the end. Look at your target groups but do not take the risk of not having the smaller customers by saying in your catering literature that you have a 50 person minimum or such. I think you need to look at each situation as its own.