The normal serving of meat is 4 ounces per person. You could reduce it somewhat because there are beans and cheese in tacos which add more protein. 4 ounces per person is 25 pounds of ground beef. Three ounces means you need less than 19 pounds.
At home I cook up 1lb of meat and it feeds three people, so I would guess somewhere between 30 and 35lbs for 100 people. It would depend on what you were serving with the tacos - if you have a lot of other dishes as well then you probably wouldn't need quite so much taco filling.