Does “DOE[USA] x6” really mean that the database has samples from 6 different DOEs?
No, each row represents one person’s pedigree, with birthplaces consolidated. The 6 is for eight generations, all in the USA. A different pedigree for five generations might read DOE[USA]x3, DOE[ENGLAND]x2. You can get the total count of records for a certain surname on this page: http://smgf.org/paternal_surnames.