Cotton number 20 is a cotton which is thicker than number 32 because the more low number the thread will be large. But if we are talking about the trendy cotton in Thailand. Cotton 32 is the trendiest because it is the most comfortable to wear. It is much more ventilated , while cotton 40 is usually used to make shirts for babies or a shirt which emphasizes extra thinness. Some brand name clothing uses cotton that is higher than cotton 40 , but these must be custom made.