unity – How to make one material on object with many parts inside?

No, you don’t need to use an unique material for each 3d model asset. It is possible for multiple meshes to share materials. The most blunt way to do that is to simply replace the materials of the assets in Unity after you imported them. You do that by drag&dropping a material from your Unity project view onto the assets.

But it might be better to import your models in a way that they already share materials. How to do that exactly depends on what software you are using to create your models. But usually Unity will automatically recognize if two models use the exact same material based on the names, settings and textures of the materials.