Xbox Series X will come with 1TB of built-in storage space at launch, but you won't be able to use all of that for storing games.
Consoles need somewhere to store their own operating system and files, and in the case of Xbox Series X we now know exactly how much room you'll have left over to play with when it arrives. IGN went hands-on with the console and confirmed that it will give users 802GB of storage space to use for their own purposes. The 1TB storage expansion from Seagate gives you another 920GB of usable space. It's quite expensive at $219 per card, but at least it more than doubles your room for upcoming Xbox Series X games.
You will also be able to store games on external hard drives, though you'll only be able to play them if they're backwards-compatible titles from Xbox One or earlier. Xbox Series X games will only run off the built-in storage or supported SSD expansion cards, though you can store them on USB drives and then transfer them over before playing.
Other impressions from the latest wave of Xbox Series X hands-on previews have largely focused on how the load times are way faster, though that probably has a lot to do with the fact that Microsoft is only letting people play backwards compatible games so far. Those previews have also confirmed that Xbox Series X supports backwards compatible crossplay; that is, you'll be able to link up in multiplayer with Xbox One players while you're playing a backwards compatible title on Xbox Series X.
Here are all the Xbox Series X launch games that will be available on day one.