For the meals on Shabat, we use two challot (loaves of challa, bread). Why two?
After leaving Egypt, the Israelites wandered through the desert for forty years. During that time they ate the manna that fell from the sky. Every day they were to collect the amount of manna they needed for that day. It was impossible to keep manna from one day to the next, because it rotted. Only on Fridays they were to gather a double portion, one for the Friday, one for Shabat. The leftovers would be good to eat on Shabat. This method enabled the Israelites to keep Shabat (gathering manna would be work).
Then said the LORD unto Moses: ‘Behold, I will cause to rain bread from heaven for you; and the people shall go out and gather a day’s portion every day, that I may prove them, whether they will walk in My law, or not. And it shall come to pass on the sixth day that they shall prepare that which they bring in, and it shall be twice as much as they gather daily.’ (Shemot/Exodus 16, 4-5, Mechon Mamre translation)
In remembrance of the double portion of manna that fell in honor of Shabat, we have two challot for the evening meal and the midday meal (some have two for every meal). They are called Lechem Mishneh (לֶחֶם מִשְׁנֶה from Shemot/Exodus 22).