It is not the number of tests, but the percentage of positives.
We keep hearing messages like, "We are doing more testing, therefore we are detecting more cases of the coronavirus." That statement in itself is true, however -- it is meant to illustrate that the reason the cases are going up is because of the testing, not the increase in the infection rate.
It is the increase in the infection rate that is the measure to track with the testing. When the percentage of "positives" goes up, no matter how many tests you do, we/you/I have a problem.
The other thing to watch for is the number of hospitalizations and the availability of hospital beds and Intensive Care Unit beds. In hot spots like Arizona and Texas, the medical systems are beginning to stretch thin.
If you recall, one of the measurements for reopening was the ability of the medical system to meet the treatment needs for the number of people sick. Already here in Washington state, we have one rural area transferring patients to Seattle hospitals due to an overflow of patients that the local capacity cannot handle.
It will be interesting to watch to see if states that reopened early or perhaps never really shut down are able to reverse course and close back down or if there is enough public pressure to do so.
It is a grand experiment with lives on the line.