X is counted by the total zoom divided by the widest zoom.
70-200mm = 200/70 = 2.8X, 1X = 70mm.
28-70mm == 70/28 = 2.5X, 1X = 28mm.
500mm = 500/500 = 1X, 1X = 500mm.
And so on.
In short, "X number of zoom", this phrase, is totally useless. The actual full range known is much better.