Why do we braid challah with six strands?
This is a common question
We braid with six because we want to have 12 pieces on the table in front of us. Why 12? Because our Shabbos table is reminiscent of the “shulchan” in the Beis Hamikdash, and then, there were always 12 loaves called “lechem Hapanim” on the shulchan. We, too, want the number 12, so we braid each challah with six, and then when we put down two challahs, we therefore have 12 ‘pieces’ on the table, just like the shulchan in the Beis Hamikdash.
Others make a challah that is literally 12 pieces, and the source of the reason is the same.