Shipping containers are normally fitted with industry standard MV303 vent.
It is very easy to have extra standard vents fitted to a container and can also be carried out on site, but having them fitted before would be a much easier solution.
Extra vents can be fitted with effecting CSC certification and are also within ISO.
Other vents can be fitted to a container with give extra air flow and are normally aluminium fixed blade louvered vents and can be fitted internally with weld wire mesh or insect mesh. These types of vents can be produced in sixes from 150mm x 150mm to 2000mmx2000mm. They are also within ISO and do not effect CSC for shipping.
Mechanical ventilation can also be fitted as part of an electrical installation.