The age-old question. And probably the worst - but best - answer a bar examinee wants to hear is: It depends. (By the way, get used to giving that answer. It's a lawyer's most obvious reply to any question.)
Having said that, some general guidelines are worth outlining.
First, and this one is far and away the most important, remember
"quality over quantity." I would rather see a student do 800 quality
questions than 3,000 questions where he doesn't learn from his mistakes,
doesn't read the answer explanations, doesn't make index cards, doesn't
do anything except rifle through a bunch of questions without a plan.
What do I mean by "quality questions"? Well, exactly the above. Doing a set of 25 or so; then reading through the answer explanations. Always the ones you got incorrect or guessed or we're half-sure and, preferably, on the ones you felt comfortable answering too. Don't rush through reading answer explanations, as if it were a check-off-the-box chore. This step is very important. Think about the answer, why you got it wrong, what trick tricked you, what law didn't you know. Then make an index card about it, so that you don't get caught in the trap next time. Also, go back to Emanuel's Strategies & Tactics (S&T) mini-outlines. Read the mini-outline for the subject in which you're taking practice questions. Always keep S&T's tips and tricks fresh in your head. They took a few minutes to review, but are worth their weight in gold.
Regarding where to source your practice questions, please read my previous review of various bar materials. In short, avoid Barbri's questions - they're not realistic. S&T questions are the best, followed second by PMBR's Red Book. Those two sources alone will yield about 1,500 questions, which should be plenty for most students. S&T questions are actual, previous MBE questions. Use them as your base of questions to really learn the law and get comfortable with MBE questions. Then integrate your S&T questions with PMBR's Red Book questions. PMBR questions are there to hone your test-taking skills and also to measure the depth of your law knowledge. These questions are tough. After a 1,000 of them, you'll be seasoned for anything the MBE throws at you. And while they're a tad outdated, don't fret too much over that. PMBR questions are still very relevant for today's purposes.
In terms of actual quantity, it varies by student. If you're comfortable with MBE law, 1,000 questions should be sufficient. If you're knowledge is average or below average, or if you struggle with multiple-choice exams, 1,500 to 2,000 questions might be better. But never more than 2,000 questions for any student. As I tutor, I will meet with each student individually to design a plan that best fits his/her available time, strengths and weaknesses, goals, etc.
Focus on one subject at a time. And remember, it's a graduated-step process. In the first weeks of studying, 25 or so questions per subject is sufficient. Then, gradually, build up to 50 or so questions before moving onto the next subject. Then, with two weeks left, start mixing subjects.
Unlike Barbri's schedule, I don't see much value in taking a full-length practice exam. That's six hours of test-taking, plus two days of reviewing each question. If you're working full-time like I was, the benefit does not outweigh the cost. One half-length exam (100 questions) will tell you the same thing - areas to focus, whether your timing's on point, etc. I would rather see you focus your energy on each day's practice set. In the beginning, timing takes a back seat to learning the law. But the more questions you do, coupled with a better understanding of the law and the MBE tricks and traps, the quicker you're timing will be. I'll repeat, for your timing to improve, you need all three: first, understand the law; second, know the tricks and traps; and third, get more practice questions under your belt. Focus on each prong every day, one day at a time.
By the end of your studying, you want to be getting 65-70+% correct on the tougher questions (e.g., PMBR). That may take 1,000 or so questions to get up to that percentage correct. For S&T's questions, by the end you want to be getting 75-80+% correct.
With PMBR's questions, I started off with lousy percentages and then gradually improved to 60-65% correct. Then I hit a wall. I stayed at 60-65% correct for weeks on end. Finally, in the last two weeks before the exam, I broke through the barrier and finished with 68-75% correct consistently. So, don't worry, getting to this percentage may take some time. It likely won't come together for you until the end. Just be patient. Don't let your anxiety get the best of you. Stick to your plan.