While "sex stories" are often consumed informally, Manipuri literature has a deep history of exploring romance and sensuality:
(1939) by Khwairakpam Chaoba Singh : A classic historical novel that remains a staple of 20th-century Manipuri prose. Jahera Manipuri Sex Stories Book In Manipuri 20
Most mainstream Indian publishers ignore the Northeast. Therefore, look for: While "sex stories" are often consumed informally, Manipuri
"The Heart of Manipur: A Collection of Romantic Stories" Manipuri Sex Stories Book In Manipuri 20
Stories like "Mahu Pubi" and "Boss ki Big Sister" often reach their 20th chapter (marked as Chapter 20), which matches the "20" in your query.