I think if you sample enough individuals (a relatively small number) you can capture the whole genetic diversity of a species. The uniqueness of each person is a combinatorial thing, think of it like this (and obviously that's a huge oversimplification): suppose humans had only 10 genes and each gene had 3 variants (alleles). Then there are only 30 different genes. But with that you can make 3^10 = 59049 different people. You only have to map the genome of enough people to get the 30 genes and you've captured the whole genetic diversity of the human race.