In most cases, except ancestry testing, all of the samples are fully consumed during the DNA testing process. The only exception are ancestry tests. After an ancestry test is complete, if there is any DNA left over, recollection is not required when more markers are ordered. However, this is not always the case. If you order an upgrade ancestry test, the system will automatically check whether there is any DNA remaining and if their is no more DNA remaining, another kit will need to be shipped out to you to collect more samples for the upgrade test.
For relationship, medical and traits tests, there will be no samples remaining after testing and if further testing is required, a new sample collection is kit will be shipped out.