I have a 64GB X, with 27GB used.
Of the use, I can easily count up,
8GB for the system.
7GB for music,
1GB of messages,
1GB of Health data
The remainder seems to be taking up by apps, both stock and my own.
I'd have to say that you'll probably something in the neighborhood of 50GB free on a newly activated iPhone X. That's kind of a guess, but I'm adding the 8GB of storage and estimating the size of the apps that are included (I'm counting Pages, Movies, Numbers etc)