How many challot should be on the table for Yom tov,for example Rosh Hashana? Two, or, one can be just fine?
The Shulchan Aruch (OC:529) paskens וחייב לבצע על שתי ככרות - Two loaves. The Mishna Brurah explains (ס"ק י) that the Mann also didn't fall on Yom Tov, so there was a double portion the day before. See both the Shulchan Aruch and Mishna Brurah inside on Sefaria.