Ok this might be real simple but is it better to have a lot of separated meshes sharing materials or trying to combine as much as I can?
I know Unity does batching for smaller meshes but is it also better for the occlusion culling?
Combined meshes are worse for culling when they are widely spread and not likely to appear in view at the same time. Since culling is decided by the object’s bounding box, it is possible in this case for the whole mesh to be rendered even though no part of it is close to being in view. However, you will definitely improve performance by combining close groups of objects into a single mesh each.