The positivity rate is a better measure than counting the number of tests. It represents the fraction of tests that come back positive. The World Health Organisation has advised that before easing any lockdowns the positivity rate should be at 5% or below for at least 14 days.
There are two ways of decreasing the positivity rate: either by decreasing the number of positive tests or increasing the number of tests performed. A comprehensive testing program allows a government to identify as many cases as possible and take the appropriate action.
The U.S. had an overall positivity rate of 10% on 27th July, but states such as New York were at 1.09% but in Arizona here there was a large outbreak it was over 20%. The high rate showed that the virus was spreading and the amount of testing needed to grow to help control the spread.