You can have multiple material (up to 8 I think)
Each material can have it’s own set of textures (you can reuse them between the different material)
A material can have 0 texture (and use instead the material parameter)
But if you plan to use a map, you need to at least have a Diffuse.Color if you want to use another one like a normal map.
You can use only one UV layout… your different material share it.
Always better to stay optimal… this means use the less texture than you can.
less there are, faster it load and it takes less resources.
So yes… on material using one set of texture that is covering all the UV map is a good way to optimize things.
Here again… it depends of the model.
for a small object this make sense. But for a building… not too much.