How much variation is there between DNA molecules from the same organism
The phenomenon is called mosaic single-nucleotide variations (SNVs) in somatic cells and it is an active area of scientific research (e.g., Abyzov et al., 2017, Genome Res. 27: 512-523):


  We estimate that on average a fibroblast cell in children has 1035
  mostly benign mosaic SNVs.
  
  Similar analyses in adults revealed no significant increase in the
  number of SNVs per cell, suggesting that a major fraction of mosaic
  SNVs in fibroblasts arises during development.