As many tests as possible. That is going to differ by person, hydration, kidney health and a variety of other factors as well. A person could perform as few as one test too. Science typically avoids doing only a single test though because performing multiple tests allows a scientist to look at averages and minimize the potential effects of scientific error. In addition, a good test is a repeatable test. So if a medical professional tests for urea amounts but is unable to do the test again, those initial results are not super reliable. They might be 100% accurate, but without a second test, the results cannot be confirmed. So at a minimum, two tests would be needed for accurate results. More tests just means more reliable data.