With A sized paper, however many times you double the short edge or half the long edge, it will always have the same width to height ratio. For example, the dimensions of A4 paper are 210x297mm, and half of A4 equals A5, and double A4 equals A3.
In this regard, how many sheets are in a Double A?
500 Sheets
One may also ask, what paper size is twice A4? An A3 sheet measures 420mm x 297mm. so to move up a size 210 x 2 = 420. This gives an A3 sheet the flat size equivalent of exactly 2 A4 sheets.