As I recall, Bach grew up in a very rural part of the country.  There
were no "name" teachers, "name" schools, or "famous" musicians.  But people
always discount rural education, it seems to me.  Clearly Bach had lots of
family who were experienced musically, and the part of the country where he
grew up had a lot of musical experience within their churches and family
get togethers.  It is possible that the actual musical experience was even
greater because of the limits to social life and entertainment.

Clearly, Bach left his childhood home well trained...  He made a difference
from the beginning, and they would have laughed him out of town as a
country bumpkin if he had no talent or poor musical knowledge.

Ray Bayles