Why is it necessary to have two challahs at every Shabbat meal?

The obligation observed throughout all Jewish communities is to have at least two challahs by each Shabbos meal.

See here in Shulchan Aruch Harav, Orach Chaim 274:2 (you can find there all relevant footnotes/sources as well).

About the Levush and a custom to have three - is it possible that he was referring to the custom to have a three-braided challah , i.e., that the way challah are typically baked and fashioned in Ashkenazi communities is to start with three distinct braids, and then to weave them together. Reply

You are most welcome and thank you for looking it back up and clarifying. Shavua tov to you as well! Reply

The tradition of having two challahs on the Shabbat table applies regardless of how many people are in attendance. However, you don't need to eat both loaves. Reply