It took me about 20 lessons. It varies on the amount of driving experience you've had. eg. A few of my friends grew up on farms and they only had about three lessons.
There is no set answer to this, although 20 sounds low to me. I passed 2 weeks ago, at the second attempt, and I totted up 48 lessons over 2 years. You should not enter for the test until YOU are ready, and your instructor also feels you are. Make sure you are comfortable behind the wheel and that you are constantly observing - especially for bikes which can sneak up on you in the wing mirror / blind spot in a milisecond. I wish you the best of luck!
I had 18 lessons and past first time. And I was rubbish when I started (at 23). Make sure you get a good instructor. If I can do it in 18 lessons, anyone can!!
I recall that someone used to say it was your age + 5 but obviously it's sometimes better to learn young and get it over and done with.
Personally I've never agreed with the intensive driving course type ones as I'm still "learning" new things about driving on the roads after passing my test over 7 years ago!
It took me three lessons - but my mum was a driving instructor and she used to pick me up from school when I was young whilst teaching a pupil. Probably wouldn't be allowed now!