im having a dinner for my DH for his b-day next week. I was going to do an all out bbq but i want to make it easy on myself so i am going to just bbq steak

i am making some chips and dips
ordereing rice balls and pot croquettes
nice big salad
sausage potatoes and peppers

i wanted to make flank steaks since i can bbq and slice them up how many lbs would you get for 21 people?