How much insulation do I need for my van?
Van insulation best practices
Casting aside complexities, the easy answer is to say as much as you can fit in the space. The structure is a metal box; it changes temperature rapidly. Mitigating contact between the living space and the exterior elements is paramount to an enjoyable environment. Insulation will help dramatically and should be placed across the entire thermal bridge for the best results. That said, there is a chart with suggestions that draws from our experience in the space on the Havelock Wool #vanlife page.