The number of chutes differs mostly based on preference.
You can have just 1 chute, which you attach to the preheater that you are taking the melted material from. As the chute is not permanently attached to the preheater, you can then which it over to another preheater.
However, for your convenience, you can get 1 chute for each preheater you are using. This is especially recommended if you are working with different colors.