Reading the 1938 Om Mandli document, I noticed the early Brahma Kumaris being called "Sakhis" and researched it.
I wondered if this was the answer where the idea of "The 8" comes from ... your opinion or any inside information welcome.
Eight Main Gopis (Asta Sakhis)
These eight are the leaders of all the other gopis. They always serve Radha and try to please Her in every way. They are the dearest and most beloved friends of Srimati Radharani. Rupa Gosvami describes the qualities of the eight main gopis in Radha Krishna Ganodesa Dipika.
- 1. Lalita Sakhi
2. Visakha Sakhi
3. Campakalata Sakhi
4. Citra Sakhi
5. Tungavidya Sakhi
6. Indulekha Sakhi
7. Rangadevi Sakhi
8. Sudevi Sakhi
She is Srimati Radharani main competitor for the attention of Krishna. Srimati Radharani is her cousin. Candravali is the daughter of Candrabhanu, who is the brother of Vrishabhanu. She lived in the village of Sakhi Sthali near Govardhana Hill with her husband Govardhana Malla.
Out of the hundreds of thousands of gopis, eight are considered most prominent. They are: Radharani, Candravali, Lalita, Vishakha, Padma, Bhadra, Saibya and Syama. Of these two Radharani and Candravali are considered most dear. Of these two Radharani is the most dear to Krishna.