Wind speeds in the upper atmosphere can be very fast. These balloons will not be stationary. Based on my limited knowledge of the project, the only way they could have consistent coverage would be to have evenly spaced balloons around the Earth. That assumes that wind currents would cause the balloons to travel around the Earth in a dependable pattern. Considering the circumference of the Earth 40,008km and 80km coverage diameter, 40008 / 80 = 500 balloons would be required at minimum if all the balloons traveled at the same speed and in a dependable pattern. I would imagine the wind currents the balloons would travel on would be greater than the Earth's circumference, so more than 500 balloons would be required. Also, I doubt the balloons could maintain a perfect 80km spacing. I can think of other factors that would make dependable coverage difficult.