At this time, you are not able to renumber the GradeCam forms. You can create forms for 50, 75 or 100 and laminate to reuse those forms. Then you can give zero point value to any of the questions that you do not want to grade like to 1-24 and then grade 25-50. You do not want to write inside the bubble form so you would not want to cross off the numbers on the form if skipping any numbers.
Hope this helps. We have had discussion on being able to renumber but have not decided on when or if to add this feature.