This is one of the most common questions asked by people learning to drive and is one of the most difficult to answer accurately. This is because the number of lessons someone needs is affected by their ability, commitment, practice and the ability of their instructor. There are people out there on the roads who have had no lessons at all and some who have had more than 100. In my opinion, the former is far too few and the latter is way too many. Furthermore, the more attempts at passing your test, the more lessons you will inevitably end up having.
So how many is too many? Well if you consider that the DSA say that the average is around 45 hours of tuition, coupled with 30 hours private practice, then that gives a really good indication. Your instructor should always be able to give you some idea of how many lessons you will need but the actual number may change as you work through the syllabus. As a guide, you should be aiming to complete your training and pass your test within the 45 hours. If it doesn't look as though you are going to make it, then you should talk to your instructor about your progress.
If you are still not happy, then take an assessment lesson with another instructor and see if they can give you an idea of the progress you are making. Do bear in mind that your instructor is (or should be) a professional and therefore is best placed to assess your driving and identify when you should take your test. However, you are the customer and as such you should take ownership of your learning process and ensure that you are getting value for money and are not being taken for a ride. It is a difficult judgement to make sometimes but the overriding consideration should be this - if you are not entirely happy with your instructor's approach, then do something about it.