For a regular Shabbos: 2 Challos of 6 strand braid each equals 12 representing the 12 loaves on the shulchan. I have made pull-aparts with 6 or 12 balls each (much easier than braiding).
From Rosh HaShana through Hoshana Raba we eat round challos to symablize to cirlce of the year/life. Raisins symbolize a sweet new year (some use a honey glaze or a streusel topping for the same reason).
Bakeries make round challos for all yomim tovim because its faster!!
Everthing however, is based on minhag, not halacha.