I have 32 GB Ram on my computer, how can i make Blender use all (or at least, the most) of the RAM when rendering?
Memory is for storing information. If blender doesn't have 32GB of information to store, then it won't use up 32GB of memory.
To force blender to use more ram than it needs would mean reserving memory and not putting anything in it (a waste), or filling it with useless data which doesn't help blender render (also a waste).
TL;DR: This doesn't really seem like it would be useful.