Due to the unique design of the new iPhone X, it was not possible to include the battery percentage indicator on the top. iPhone X has packed lots of other goodies on the top center notch. So due to space constraints, Apple decided not to include the setting to enable the battery percentage indicator.
but not to worry about it, Apple has already enabled that setting by default in the iOS.
Just swipe from the top right side of the iPhone X to pull down the control center, there you will see the battery percentage indicator in its full glory.
Here is a video showing same.