Answer:
Assuming that they are not all the same height: 0
This is because the shortest person in the line will have to have someone taller them than on at least one side of them.
If they are all the same height: the number of ways that they could stand in a line is
7 x 6 x 5 x 4 x 3 x 2 x 1 = 7! = 5040