You're going to have a tough time finding a monitor suitable for the xbox series x at this point in time. The consoles come with HDMI 2.1 as the other member listed above there but there is 0 monitors currently being sold that come with such hardware.
There is a couple companies in the production stages as well with not a lot of information; there is the company "Eve" which is going to be selling a 4K144hz monitor with hdmi 2.1; it's in the pre-order stage and it almost sounds too good to be true since it's asking for $669 USD another note is apparently this same company made a microsoft surface tablet a couple years back and some customers reported they didn't even get them sent to them and are having troubles to this day getting refunds.. So that monitor I don't think I would suggest..
Asus also announced they're coming out with a monitor I think maybe in the next couple months, or more than likely early 2021.. I would expect the other brands to follow along with that and we'll see some HDMI 2.1 monitors in the early of 2021. If you want something now all you should be looking for is something thats 120hz and can do that over the hdmi, and most likely you'd want a Freesync monitor.
A couple monitors I would suggest checking out are the LG 27GN850 or the Dell 2721DGF they're a Freesync 1440P 144hz monitor, HDR, IPS Panel, hdmi 2.0, and I have heard nothing but good things about them. The Dell 27271dgf actually uses the same panel as the LG but it's a 2020 monitor vs a 2019 monitor they're just on back order right now. I'm not sure where you live but for me in Canada the LG monitor is 699, and the Dell one is 739.
Ultimately it comes down to what you'd want in a monitor and I am personally thinking about grabbing one of the two I mentioned above, but it would also make sense to make look into a cheap freesync monitor that can hit the 120hz over hdmi. I would also like to note a monitor with HDMI 1.4 is able to support 1920x1080 at 120hz but i'm not sure how reliable it is, you be better off with HDMI 2.0 over 1.4 as it can support higher resolutions and that 120/144hz mark.