No, they can't fit on the same car.
5 (4 plus the spare).
5 Tires on a Car (including the spare).
If by spare tires you mean the temporary spare that is in the trunk, then a car dealership is your best bet. If you mean tires to keep as a back up, the I can recommend Tony's American Car Care Center in Renton. ACCC is a nationwide franchise, so you will be covered if you are travelling.
Assuming that the car has four tires, those four will wear at roughly the same rate, so you would have to replace 4 tires at 36000km, leaving one as the spare for the next set.
It doesn't have to but it would be wise.
Not recommended. All tires on a vehicle should be of the same type.
It is under the floor in the trunk unless the car has run flat tires. In that case you will not have a spare tire.
It is under the floor in the trunk unless the car has run flat tires. In that case you will not have a spare tire.
For SAFETY and HANDLEING YES.
Check the pressure in all five tires-spare included. You will have to remove the spare cover, which takes some time if you have a soft cover. Once all the tires are at the same pressure, the light should go out. I did it with the car parked, so it seems driving it for reset is not required. If the pressure is off between the tires after adding air, the light may stay on. They must all be close or the same for the light to go out. Use a real tire gage rather than the one at the gas station air nozzle.
It is not illegal. Some cars do not have spare tires nor a place to store a spare. Example would be a new Corvette.
I check the tire air pressure in all the tires including the spare every oil change or before a long trip.