12mm ply inside wardrobe is used for making back portion of your wardrobe. Usually for the verticals 19mm Ply is used. Verticals are goign to take lot of weight of wardrobe so 19 mm is recommended. However if you are not goign to fill lot of heavy stuff inside your wardrobe, 12 – 15mm is also sufficient. More then 12 mm or 19 mm the important question is about whether to use Marine ply or commercial ply
In wardrobe you have two options in choosing PLY ( wood ) & that pretty much decides how much will be the cost of the furniture. It could be marine Ply or commercial ply. If Budget is not an issue & you want THE BEST then marine is your option. However if you are renovating your home within limited budget, may be you want to go for commercial ply for wardrobe. Limit use of Marine only within area where water can come in contact ( kitchen shutters below sink for example ) .
Marine ply of a good quality will give you a furniture which will last for 10-15 years, may be more. Commercial will last less longer . Marine is water proof, termite proof – commercial is not.
In old days people believed in longevity – your dadaji kept a small almari for 50 years. Today, not everyone wants to make a furniture which is very expensive & last for for your entire life. However is there is white ant or termite problem, marine ply is your option.