Our parish's burial register goes back to 1559. There are just short of 10,000 entries in our registers, and burials would have been taking place for many centuries before 1559. It's only in the 20th century that records were kept of where the graves are located; we have around 700 headstones, some of which were moved to the margins of the churchyard under a Faculty in the later 20th century. It's a fair guess that 15,000 are buried in our churchyard and we know the locations of only a few hundred.
While numbers will differ, I'm sure the general principles will apply to most rural parsh churchyards.