In terms of design, the iPhone Xs looks more or less like its predecessor from last year, the iPhone X. It features a similar design, a screen with exactly the same dimensions, and a dual camera setup on the back. This means that technically you should be able to use cases for the iPhone X with the iPhone Xs.
Unfortunately it seems that might not be the case for all cases. According to a report from Japanese publication Macotakara, it appears that the iPhone Xs has a camera bump that is slightly larger compared to its predecessor. It measures 25.5mm by 11.27mm, versus the iPhone X whose camera measures 24.13mm by 11.26mm.
Now to the naked eye the differences are imperceivable, but what this means is that cases that have been designed to be a tight/exact fit for the iPhone X will not fit the iPhone Xs, at least not properly. We suppose depending on the material, you could try and squeeze it in, but your mileage may vary.
In any case it’s something to think about so if you are in need for an iPhone Xs case but can only find iPhone X versions around, just make sure that there is room in the camera cutout for it.