There is a rule of thumb that on average a learner will need one and a half hours of tuition for every year of his or her age. This may work for you, but if you're not "average", you might learn faster or slower than this. In reality, the number of lessons you will need will depend on these factors:
Your age ....  Although one's interest in learning may actually increase with age, unfortunately one's ability to absorb a new skill probably tends to decrease. However driving is a skill that can in time be acquired by just about everyone, whatever their age.
Your natural aptitude for driving ....  Just as some people are "better" than others at, for example, crossword puzzles, languages or sport. It might take you longer to learn to drive than your friends or relatives.
Your previous experience of being driven and/or of driving ....  Even if you have never driven before, it is likely that you have been in a car driven by somebody else. The way that person drove may have a significant influence on the way you drive, as humans learn naturally just by observing someone else's actions. You may have watched a "good" performance which may help you make rapid progress with your lessons.
If you have previous experience, either of driving a car on private land, or better still, of riding a moped on the road, this may well reduce the number of lessons you will need.
Any other driving you will be doing outside your driving lessons ....  In general, the more experience of driving you get, the quicker you will learn. So if you have the opportunity to drive legally with relatives or friends, take it. You should note however, that if you have never driven before, it is best to delay any extra driving until you have sufficient skills to control a car with minimal assistance, and have enough knowledge and judgement to get around safely. Check out the DSA website for motoring