Where do you live? Or at least where are you buying it?
Taxes, at least in the U.S, vary depending on State.
Whatever your local sales Tax is, add it to the price of the iPhone.
In the U.S. by the way, the 256GB iPhone X is $1149.00 add the tax to that.
You can also go through the motions of buying the iPhone X on the Apple's online Store , and it will give you an estimate based on Zip Code when you add it to your shopping cart / bag.