Yup. It's a practical skill, so the more you drive (up to a point) the better you will get. And going it alone means that there is no-one else to back you up so you'll have to concentrate more, and thus improve.
Personally i don't think it's about the quantity of lessons more like the quality. I've had three driving instructors...the first wasn't qualified (i didn't know that at the time) and she never told me really basic things like changing to a lower gear when approaching junctions or slowing down...therefore i would always stall everytime i came to a roundabout or junction and i though it was because i was crap. I was never really able to improve my driving much becuase i never knew basic things like that...and i had about 40 hours of lessons with her.
My second instructor was qulaified but he was an arse...he told me things (like changing gears etc) that my previous intructor didn't...however he didn't explain why i had to do it...e.g. i knew i had to check my centre and left mirror if i turned left...but didn't really understand why.
My instructor i have now is amazing. I've had a few lessons with her and after the first hour my driving improved dramaticly. She makes sure i understand everything...constantly asks me questions about it. I think if i had her from the start i would have passed my test ages ago by now.
So basically i really think it's not about how many lessons you have because if you have a crap instructor you'll still probably be a crap driver...however if you have a good instructor and have decent lessons then you will probably become a better driver in the long run.