Will there be wine service at the party? I wouldn’t be comfortable having 1 server to serve and cocktail for more than 20 people in a four course dinner. Even then, I would want to make sure the first course was pre-set, in addition to bread, butter, coffee cups and saucers, water and iced teas for half the guests if the bar is a cash bar or if there wasn’t a bar. If it’s a free bar, I wouldn’t worry about the iced teas.
Even 20 people all at once for one server is stretching yourself thin, except that you have 1 busser for every 3 tables. That helps.
How is your service station set up for this party? How far will servers have to walk to fill and refill water pitchers and coffee pots? Where is your bus station in relation to the tables, or do you use large oval trays?
How you service this large a party will have a lot to do with how they view your prices. It will definately affect whether you get referrals.
In my opinion, it’s worth it to go above and beyond on service to make a name for yourself, and build value in your banquet prices so you can increase them when you need to. With that in mind, in my years running country clubs, served dinners with more than 2 courses and cocktail service, never got less than:
1 server per 15 guests
1 busser per 50 guests
1 bartender per 75-100 guests
1 manager/captain per 150 guests
1 buffet attendant per 75 guests
There are many restaurants and catering services out there that don’t offer anything beyond food and food table setup. They set things out, and people help themselves. This is all fine for a family reunion, and casual affairs with paper and plastic. If you’re plating and serving, provide the same level of service you would in a restaurant, and make a name for yourself.