Can a woman wear a single Thirumann or should they wear an extra small Thirumann on the forehead below the parted hairline, on their neck and hands? Few elders say if women keep vermillion on their necks, the wealth of their maternal home shall decline. Would like to know about this as preached by Acaryas and Swami Desikan; kindly elaborate.

While doing Samashrayanam, the Acarya will instruct the women to adorn 12 Thirumann kappu and preach the respective 12 divine names.
It would be difficult to wear 12 Thirumann every morning due to household chores, so it is not in practice, but it is not wrong to wear 12 Thirumann every day. Elderly women also wear only one Thirumann, hence we follow the same.