Yes, you can add more pages if you wish.
At this stage, the maximum of pages per book is 100 pages plus one cover page. If you wish to have a bigger book, you need to split your project into two projects. How? Look here: https://mybooks.zendesk.com/entries/23685863-How-can-I-split-my-project-into-two-books-
Note: there will be a charge for extra pages at the checkout.