It's a few years back now but I reckon I had about 50 in total!! Yes, that sounds a lot but 40 of those were with Pennine School of Motoring. Ripped me off chronically and learnt sweet fa. Failed first time after 40 lessons with them. Decided to then go with BSM , got a proper instructor and only needed another 10 lessons to pass second time. To answer your question, with a decent instructor it shouldn't take too many to put in for your test. However, like a lot of things in life, it will vary from individuall to individual and it's a question really only your instructor can answer - and only then if he is honest and doesn't work for Pennine School of Motoring!
Was told to put instraight away as tests were a long time in waiting (1991), test came back in 3 weeks, told to keep and see what happened. Upped my lessons to 3 a week for 2 weeks and passed first time after 8 lessons. Probably lucky as didn't get clutch control until lesson prior test!