18 parvas in Mahabharata

18 chapters in Bhagavad Gita

18 days of fighting in the war

11 + 7 = 18 total akshauhinis fighting the war from both sides

21870 elephants + 21870 chariots + 65610 cavalry + 109350 foot soldiers = 218700 total units in an akshauhini.

The sum of all digits in every number above is also 18.

18 literally means Jaya (the original name of Mahabharata) or Victory.