When you multiply the size of the human genome by the mutation rate per site per generation you get 35 mutations per genome per generation. How many cell divisions occur in one spot in the human body in a 20 year generation? If it is significantly greater than 35 than you don't have a mutation roughly occurring everytime a cell divides. For example if one spot in the body has cell division occurring 3500 times in a 20 year period, you have mutations occurring one time in every 100 cell divisions.
"Here we present, to our knowledge, the first direct comparative analysis of male and female germline mutation rates from the complete genome sequences of two parent-offspring trios. Through extensive validation, we identified 49 and 35 germline de novo mutations (DNMs) in two trio offspring, as well as 1,586 non-germline DNMs arising either somatically or in the cell lines from which the DNA was derived."
Europe PMC
I think it would also be safe to say that you could find more mutations by increasing the tissue types you are sequencing, as well as the age of the donor. However, it doesn't appear to be a crazy number, like 1 million or so somatic mutations.
Edited by Taq, : No reason given.