Depending on what you want to do you might either want to record demos or actual videos. The big advantage of a demo file is, that it is not an actual video and you can replay the demo in modified jk2 versions (like joMME) to watch a scene from a different point of view. You can look at the people behind you or fly around a corner to see who was hiding there.
To record demos in jk2 you have to set "g_synchronousClients" to 1, use the command "record <filename>" and set "g_synchronousClients" to 0 again (if it is set to 1 you might experience extreme delays when moving etc., depending on your connection to the server).
With jk2mv > 1.0.2 (not officially released, nigthly builds [work in progress, could be crashy or behave otherwise unexpected] can be found at
https://jk2mv.org/builds) you don't need to set "g_synchronousClients" anymore.
If you are looking for a demo script that automatically records with different names each time you press a hot key, I don't have one, but I could ask around.