A credit is deducted every time a candidate finishes the first test of an assessment. So, if a candidate drops out of an assessment before finishing the first test, the credit won't get deducted.
For example, you create an assessment with two tests and invite 10 candidates to take the assessment. 2 candidates drop out midway through Test 1, 3 candidates finish Test 1 and drop out during Test 2. 5 candidates finish both tests. You will then be charged 8 credits.