You will need data like final page count, trim size, and markets before you can assign a particular ISBN to your book. (peggydekay.com)
Remember to confirm the final page count after the text has been formatted as page count's often change. (selfpublishing411.com)
You'll need a final page count for your cover, but now is a good time to be thinking of ideas and a general concept for your cover. (indiepublishinggroup.com)