Your misunderstanding the use of the maintenance facility, it does not make supplies, it prevents ships from using supplies.
Like in the description, 20 maintenance facilities can support ships of 4000 or less, so any ships of 4000 tons or less won't get damaged and use maintenance supplies.
Imagine they are a place that ships can dock and turn off and not get damaged, leaving a ship in the middle of space can cause failures and make it use maintenance supplies.