Each book has an additional question bank of 25 questions online - there's a code in the book that you enter online to get access. (If you have any tech trouble, email
studentservices@manhattangmat.com or call 800.576.GMAT from the US and Canada.)
You should also supplement our questions with the Official Guide (real past test questions) - the real thing is always the best practice! The most recent main book is The Official Guide for GMAT Review 13th Edition (or OG13) and has ~900 problems that you can put into problem sets. :)
You can also get an additional 90 free real questions via the GMATPrep 2.0 software.