If you are printing labels from a mailing list (meaning that each label will be different), then the number of pages will depend on the size of your mailing list and should be automatically generated thanks to your spreadsheet.
Now if you are printing a page full of the same labels, you can choose to print several copies of the same page in order to get more pages.
Here is a detailed tutorial with a quick video at the end to better help you: quick tutorial link
Let us know if this helps!