It is customary for every household to bake Challahs for Shabbos which are used for Lechem Mishneh and not to buy them at the bakery as is done during the week. This matter of baking one’s own Challah is included in the honoring of Shabbos and Yom Tov and one is not to divert from this custom.
How much is one to bake? One is to bake at least the amount that requires one to remove Challah from the dough.
Pas Akum: (For those which are accustomed to eat Pas Akum throughout the week it is proper for them to refrain from eating it on Shabbos and Yom Tov. Rather they are to eat from the Kosher breads which have been kneaded in their homes, as this is included in the Mitzvah of honoring Shabbos and Yom Tov.)
One is to bake Challahs for Shabbos as opposed to buying them from a store.
Why today are not all women particular to bake Challahs for Shabbos and rather they buy Challahs from the store?
Some Poskim rule that today being that fresh and tasty Challahs are available in all bakeries it is not necessary for every woman to bake Challahs in their home. This especially applies if there is much work needed to be done for Shabbos in the home and it is a short Friday. Other Poskim however argue that even today one may not divert from the custom of baking Challahs in the home.
May one bake only a few Challahs and buy the remainder from bakery even according to the above custom?
Yes. One may use some home baked Challahs and some bakery Challahs even according to the above mentioned custom.
Is one to bake the Challahs specifically on Erev Shabbos?
 See also Shach Yoreh Deah 324/25 that the women are scrupulous to specifically bake enough to separate Shiur Challah on Erev Shabbos.
 Gentile baked bread
 As for why this law was placed in parentheses by Admur, see Chikreiy Halachos 3/30
 Oar Letziyon 2/47; Mishnes Yosef 5/63
 Mishneh Halachos 15/95
 Chelkas Yaakov 1/59
 Seder Hayom ; Siddur Yaavetz; Machatzis Hashekel 242/10
 As amongst the reasons for baking Challahs for Shabbos is because it resembles the showbread which were baked on Erev Shabbos as well as that it rectifies the sin of Adam which was the Challah of the world.[See Siddur Yaavetz; M”B 242/6]