Numbers such as 7, 9, 11 and 13 also represent notes along that same scale. As these go beyond one octave of the scale, just continue counting upwards that same scale. However, there is an exception: 7 is presumed flattened a semitone unless otherwise directed. For our A major example the numbers 7, 9, 11, and 13 would correspond to the notes, G [natural], B, D, and F#. If the chord name only contains a single number such as "11" it is presumed that the numbers that precede it are also included. Thus for an 11 chord, you also play the 9, 7 and root triad: A, C#, E, G, B, D. If you want the 11th scale degree exclusively you would write "add 11".
Also, A11 is a pretty rare one.. 'filled out' dominant 7ths with a natural 11th sound pretty gross, so its quantum's assessment that #11 is a possibility seems quite likely.