ZipGrade will only grade the questions that have an answer defined on the key. Percentages are based on this same number. Therefore, you can create any size quiz of up to 100 questions. And the 'active' questions can be anywhere on the answer sheet.
Some teachers use this feature to re-use an answer sheet multiple times. Use answers 1-10 for monday's exit ticket and then questions 11-20 for tuesday's exit ticket.