Six is most often the number of a completed work, and it is completed rather than final: the traditions that use it that way almost always have a seventh standing outside the count.
Genesis makes the world in six days and rests on the seventh, and the whole Jewish and Christian week is that arithmetic turned into a calendar. The Mishnah is arranged in six orders. Jewish liturgical thought also counts six directions, four horizontal and two vertical, with the worshipper at the centre making a seventh position that is not a direction.
Buddhist cosmology counts six realms into which a being may be reborn, from the gods down to the hells, and the earlier list of perfections a bodhisattva cultivates is six before later texts extend it to ten. Six is also the number of lines in a hexagram of the Book of Changes, which is why that system has sixty-four figures and not some other number.
The six-pointed star is a special case worth stating carefully, because it is usually told wrongly. Two interlaced triangles are an old and widely distributed decorative motif with no single meaning, appearing in Jewish, Christian and Islamic contexts alike. Its adoption as the specific emblem of Jewish identity is a late development, traced by Gershom Scholem to the early modern period and consolidated only in the nineteenth century.
For the Greek arithmologists six mattered for a different reason. Its divisors, one, two and three, add to six exactly, which makes it the first perfect number.