When serving 30 people with mashed potatoes you need at least 15 lbs of potatoes to make enough mashed potatoes for 30 people.
It's even better to make slightly more than you would need and then you can save the leftovers for later.
But usually cooking at least 15 lbs of potatoes into mashed potatoes will be enough for 30 people to eat.
Good foods that go good with mashed potatoes are.
Bread, Chicken, Catfish, Beef Wellington, Steak, Meatloaf, Meatballs, Chicken Gordon Bleu.
Other foods that go well with mashed potatoes are potato croquettes, Salisbury Steak, Fried Chicken and Gravy, Steak and pan sauce.
Roast Chicken.
Potato Pierogi.
Fish Pie.
Grilled steaks
Sausages (grilled or boiled)
Ground lamb (as part of a Shepherd’s Pie)
Chicken in some sort of sauce (Chicken a la King, Chicken Francese, etc.)
Stews of any kind of meat (beef, pork, chicken, etc.)
Fish filets (usually in a Beurre Blanc sauce)
You can also top the mashed potatoes with some gravy and hamburger.
Or even top the mashed potatoes and gravy with some nacho cheese.
You could also dip nacho chips into the nacho cheese and gravy mashed potatoes.