It's impossible to tell, in isolation. This code does work:
tell application id "com.adobe.illustrator" -- Adobe Illustrator
tell document 1
set theCount to count of artboards
repeat with i from 1 to theCount
set {x1, y1, x2, y2} to artboard rectangle of artboard i
set y1 to y1 + 20 -- however much you want to add at top
-- place item based on the bounds
set artboard rectangle of artboard i to {x1, y1, x2, y2}
end repeat
end tell
end tell