We make 1/4 lb patties & my husband usually eats 2. So minimum for 1 adult male & the rest only eat 1 patty: it's 1 1/4 lbs minimum but our patties are never perfect
so more like 1 1/2 lbs. If it's all adults who eat 2 patties then it's closer to 2 1/2 - 3 lbs.
If you're trying to save $ on meat, I've been known to add quinoa to meat dishes to extend portions. Never tried it with hamburgers but we don't eat them often.