If you look on the product pages (the web page where you order), you will usually find the number of pages in a PDF product. However, we don't have all the web pages updated yet, so please email us at support@welltrainedmind.com and we will get you the info you need.
In most cases, you will print all (or most) of the Student Pages. The exception is The Story of the World Activity Book / Student Pages. In this case, you pick and choose according to the ages and interests of your students.