The amount of taco meat you need for 40 people depends on how many tacos you plan to feed per person of those 40 people.
Assuming you serve each person 1 taco then you'll need at least 2 ounces of taco meat per taco.
So if you make 40 tacos then you'll need at least 80 ounces of taco meat or beef to make 40 tacos.
However if you serve 2 tacos per person then you would need 160 ounces of taco meat for 40 people.
So as long as you figure between 2 ounces to 2.5 ounces of meat per taco and you multiply that amount of meat per the amount of tacos that you're serving people then you'll get the right amount of meat.
It's better to have a little extra taco meat than too little meat.
Whenever I plan on serving more than 20 people then I always just buy beef and taco ingredients in bulk and have any leftover saved to eat or feed my own family later.