Why there are 18 chapters in Bhagavad Gita?

There are 18 chapters in Bhagavad Gita.

There are 18 parvas in the Mahabharata.

There are 18 Mahapuranas.

What is the significance of the number, 18 ?

Even though we broadly call it Atma (soul), 18 forms of Atma have been described in the Vedas. 

1. परात्पर पुरुष – Paratpara Purusha
2. अव्ययात्मा – Avyayatma
3. अक्षरात्मा – Aksharatma
4. क्षरात्मा – Ksharatma
5. शान्तात्मा – Shantatma
6. महानात्मा – Mahanatma
7. विज्ञानात्मा – Vijnanatma
8. प्रज्ञानात्मा – Prajnanatma
9. प्राणात्मा – Pranatma
10. शारीरात्मा – Shariratma
11. हंसात्मा – Hamsatma
12. वैश्वानर अग्नि – Vaishvanara Agni
13. तैजस वायुः – Taijasavayu
14. कर्मात्मा (यज्ञात्मा) – Karmatma (Yajnatma)
15. चिदाभास – Chidabhasa
16. चिदात्मा – Chidatma
17. श्री लक्षण – Shri Lakshana
18. ऊर्क् लक्षण – Urk Lakshana

The above scriptures are meant to understand the 18 forms of Atma. That’s what the number 18 indicates.

Ramaswamy Sastry and Vighnesh Ghanapaathi