Im thinking about buying a Samsung S10e. How long can i expect to get security updates and other?
Well basically as long as possible, but you already knew that 
Sorry I am not the right one to answer, it would need a check on what kernel version it has, that often gives a good clue as to how far it can be taken to the future.
Wow that was hard to do but I confirmed the codename for the device is beyond0 (Exynos) or beyond0q (Snapdragon). With it being released in 2019, it is already 6 years old so I wouldn’t be certain how much longer it would be supported.
Also, just confirming you know that having VoLTE support for custom ROMs with Samsung devices is not easily done (or currently possible in many cases)? Are you in a country where you would need VoLTE?