Each of the three Shabbath meals require two loaves of bread. The loaves must be whole, in memory of the double portions of Manna that came from heaven on Friday for Shabbath (Lehem Mishneh.
If women are eating by themselves, they must also use Lehem Mishneh (two loaves of bread) as mentioned by the Posqim.
According to Rabbenu the Ari z"l, in the Sha'ar Hakkawwanoth, however, each of the three Shabbath meals requires twelve loaves of bread to be placed on the table. Six are placed on the table as in the diagram (face down) with the other six placed on top of them (face up), so that there are six pairs of loaves on the table back to back. Clearly this works best with flat breads.
(See Ben Ish Hai, Shanah Sheniyah, Parashath Wayera, Oth Teth Waw)
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