Rich has such a good point: QUALITY of study is better than QUANTITY. Nothing is more common among low-scorers than a situation in which they have answered hundreds (if not thousands) of GMAT questions with little to no improvement. Always study with a pinpointed purpose, and remember that analysis is your best friend.
So, how many GMAT questions do you need to answer? Only as many that will allow you to:
1) Learn to recognize how the GMAT tests each concept.
2) Apply your strategies to easy, medium, and harder level questions with high accuracy.
3) Work on pacing drills so that you can answer all questions in appropriate timing.
Sometimes it's not even answering questions that make a huge score increase. MGMAT's Sentence Correction book doesn't really have *that* many questions, but its analysis is so above-par, it's invaluable to students who want a high Verbal score.
So, if you want a high Verbal score, I suggest books to begin with like Powerscore CR, MGMAT Sentence Correction, OG 13th, OG Verbal, but there's chapters in all those books that cover more than just problems.
Other books I like:
Veritas Prep's Sentence Correction - 75 practice problems
Veritas Prep's Reading Comprehension - 90 practice problems
Examcracker's LSAT Logical Reasoning - you'll want to focus in on the chapters that overlap with the GMAT, but there's LOTS of excellent, challenging CR questions throughout the book
Grockit's question-bank online and Veritas Prep's online question bank both have high-quality practice questions. I believe Veritas Prep's question bank is now free, while Grockit offers a free trial, then a monthly membership.
Good luck!
