There is not a set number of lessons that you will have to buy or have to take with any of the tutors. Normally after your first lesson with a tutor he or she should be able to estimate how many and how often you may need lessons so that you can achieve your target grade. However as you progress with your learning, your tutor will be in a better position to make abetter estimate.
Our suggestion is that in your payment to tutors always begin with payments for one or a small number of lessons until you are totally confident about your progress and your tutor.