It is measured by Android OS versions so 9 and 9.5 would be the same Android version and that's all we know. It's only one year away so wait and see what they do.
Unless Samsung decides to give devices from 2023 One UI 9.5, assume that they're gonna pull the same stunt they did with the 2022 devices (Tab S8 series, S22 series, ZFold 5, ZFlip5, A33, A53, A73 and more did not get One UI 8.5)
It's 9.0 since the rest of the S23 series is ending on 8.5; 9.5 would mark the 5th major update when the S23 series incl the FE only has 4 major updates.
The .5 updates are now being treated like major updates since they debut with a new Galaxy S.
