If you are going through a staffing agency, they can advise you on how many people you'll need for your event. If you are hiring people direct here's a good guideline, which doesn't include a chef!
The assumption here is that, in addition to the serving staff, you will be hiring a pro chef, caterer, or preparing the food yourself and the staff will assist you in serving it.
Generally, most buffet parties with a guest list of 50 require about 4 people: 1 bartender, 1 food / kitchen helper, 2 busser / helpers to tray pass during cocktails, pick up glasses, plates, and replenish the buffet.
If you are renting plates, as opposed to using nice disposables, you should have another person to help rinse off rental dishes and pick up plates around the party.
If your party of 50 people has a sit-down meal, I'd up the staff by 1 or 2 more people on the floor. It all depends on how long you want to wait for your food, and typically, the more staff you have, the smoother your event will be.
As your guest list increases by 25 people, you should consider hiring another bartender and server.