Companies may employ a number of people in many of these roles. You could have 30+ animators on a project and a large company might be working on 3-6 projects at a time. And some of the positions are unique so there may only be one or a handful of people in that position at the company.
Note: Compositing is also used in 2D to 3D conversion. Although it uses many of the same tools and skills, it’s a different task, pipeline and different goals. Compositors can work on standard visual effects shots or conversion but will more likely need some initial guidance when switching from one to the other to learn the pipeline and additional tasks.
Companies should be clear of the type of compositing they are looking for and job seekers need to confirm what type of compositing they will be doing.