Best time to start the preparation? Approximate number of questions to be attempted to qualify GATE?

Generally 4-5 months preparation is enough to have a respectable rank in the exam. January is meant for revision of your notes and practicing of objective type questions. As far as the “number of questions” is concerned, it varies with different branches, number of students taking the exam and also from year to year. Since there is a negative marking it is always good to attempt the questions, which you are sure about.