The physics behind condensation is that the metal roof cools off and warmer air full of moisture hits it. If the metal is at the dew point of that moist air, water condenses out of the air onto the metal. Metal is a great conductor of heat, it will cool off rapidly. If you are getting condensation you have a source of warmer moist air hitting that cool metal.
Where is that warm moist air coming from? I don't know, depends on your conditions. In Arkansas I had a metal roof shed that just poured water in the mornings in spring, the rest of the year it was pretty dry. We had a lot of fog in the spring. The shed floor was dirt and the stuff I stored in it could handle the water so it was not worth fixing.
So how do you stop the problem if you have a problem, not every metal roof does. You stop the warmer moist air from hitting the underside of your metal roof. That shed had great ventilation at the bottom and the top so ventilation was not the issue. In some conditions maybe more ventilation would help. Depends on what your source of warm moist air is.
The way you solve that on your house is with a vapor barrier. The sheathing holds a vapor barrier that blocks the moist air from hitting the underside of your metal roof. Insulation may help but the vapor barrier is what does the trick.