Insert a fixed amount of item in a train

Hi, I can't figure out how to insert a specific number of item in a train. My situation is, I want to make plastic and batteries outside my factory, but I need iron, copper and coal to do so, My train setup can handle 3 cargo each train. One for plastic, one for batteries, and one which will bring needed materials, but I don't want to bring a full cargo, I need 400 of each. And I don't know how to only input 400 iron, copper and coal in a single cargo wagon.

Anyone has an idea ?