Neste contexto, você pode fazer uso de "very few" (melhor do que "few" sozinho) ou "not many".
- I like a movie/TV series that very few people know (about).
- I like a movie/TV series that not many people know (about).
- Perhaps the secret to finding a job in this competitive market is by looking for work in a lesser-known field that not many people know about. (CNN)
- A big secret (=an important secret or one that very few people know). (Longman)