12 chromosomes, making six pair of homologues (each pair looks like 'l l') carry out DNA replication to create 12 pair of sister chromatid (each pair looks like 'x x'). Each pair of chromatid pair side by side at the equator (as opposed to aligning in a row in mitosis). During anaphase I, the sister chromatid are separated into separate cells creating 2 cells each with 6 chromatid. Anaphase II occurs like in mitosis creating a total of 4 gametes (sex cells) each with 6 chromosomes.
Thus somatic (body) cells with 12 chromosomes yield gametes with 6 chromosomes.