You can not add multiple pages at once into a project. Only one page at a time.
The reason for this, is ever page added increases the size of the cover, spine, etc.
Before making a book, you should have a pretty good idea of the number of pages a book is going to be, based on the research you did beforehand and maybe you will have to add a couple or 3 or 4 pages or remove them.
This is quite easy to do. On the left hand side where Page Vision shows you the small pages, just click on the "Add Page".
Once you click it, a warning box will pop up, press "OK" and your page has been added to your project.