Before revision:  Here we show that independent of the total size of population with frequent interactions among its members, about 15000 tests with randomly selected people per day  suffice to obtain valuable data about the current number of infections and their evolution in time .
After revision:  Here we show that independent of the total size of population with frequent interactions among its members, about 15000 tests with randomly selected people per day suffice to obtain reliable data about the current number of infections and their evolution in time .