Dear Rakhi
As David said there are the Veritas Prep question bank (and the question there are pretty good too). Then you have Manhattan Question Bank and Kaplan Question Bank. (these are the three best sources I know off).
You also OG, OG Quant, OG Verbal and GMATPrep questions. But I strongly suggest you do them after you have done all the concept study. They, imho, should be the end point of your preparation.
The Important point about practice question is not only finding good questions to practice with but also avoiding bad questions. Questions that are not representative of the original GMAT, test topics not tested on the GMAT, test methods out of scope of the GMAT. These questions don't do your score any good plus they mostly act as demotivates. Some lesser known companies give you these questions for to make you sign up for their services.
Now, two final things. First, the number of questions, after a certain number, doesn't matter really; its the analysis of questions that become more important. Second, I think the moderate level (600-700) questions are far more important than the so called hard (or 700-800 level)questions in building your score.
