Optimal is a bit subjective. For that “head” 3M is extreme overkill though. When you get to the point that your vertices are smaller than the pixels that are displaying them, what use are they? At that density, you should be more focused on high frequency tertiary details like skin pores and fine wrinkles. It’s a lot simpler to sculpt with a less dense mesh while creating the main forms then use some dynamic topology to add details in slowly where needed. Just because Nomad can handle those highly dense meshes doesn’t mean they are helpful to the process of actually sculpting.
“helpful to the process of sculpting” that adds to my earlier point @Hobbyist-ish - it’s not helpful if you can’t smooth the mesh because it is too dense. It also makes it difficult to repose the mesh, so all of those major moves should be done on a less dense mesh, maybe even multi-resolution. As for numbers, that is different on every sculpt, depending on the detail required.