The higher number of objects will increase draw calls which impacts performance. But, you’ll still have to split things up a bit. First think of separating things by what materials they use. Then consider if the object might be too big or detailed to get enough detail in the lightmap which would require splitting up the mesh further to get more detail in the lightmap. So for example, you might first have all of your walls as a single mesh, but might not get enough lightmap detail, so then you’d have to split it up into multiple wall meshes.
For things like props, you want to try to import just one and then place any copies you need–so like for chairs you’d import one and then place all the copies you need in the level. If something needs interaction like a door, that will have to be a separate mesh as well, possibly even a skeletal mesh to have animation.
Try to keep the number of different materials down, and try to avoid using multiple materials on a single object.