Xbox One has a hard drive of 500GBs, logically you can’t use all that space, because the system saves part of that amount of storage capacity for its own tasks (for instance, system updates). Xbox One doesn’t allow the player to check what amount of memory is free to use and what amount is required for the system.
IGN has done its homework and Xbox One has 362-365GBs of memory free for the user. The rest of that memory is reserved for mandatory tasks (native apps, system updates, etc.).
It’s a tricky business, those companies should announce what amount of memory is free for the costumer. One way or another, 365GBs is enough memory to install around 20 games.