OK - this is awful but when I make DH and I tacos - we can usually eat a pound of meat...we are pigs, I know. I would say 10-15 pounds of meat...does that seem high? I know DH can eat 1/2 pound of taco meat for dinner. Now, are you serving other items - beans, chips/dip, rice....then I would say on the lower end of 10-15 pounds.
I just asked the girl that sits behind me and she said the same about her and her husband - she makes a pound of taco meat for the two of them.