Akshauhini

An akshauhini ( "akṣauhiṇī") is described in the Mahabharata as a battle formation consisting of 21,870 chariots (Sanskrit "ratha"); 21,870 elephants (Sanskrit "gaja"); 65,610 horses (Sanskrit "turaga") and 109,350 infantry (Sanskrit "pada sainyam") as per the Mahabharata (Adi Parva 2.15-23). Thus one akshauhini consisted of 218,700 warriors (not including the charioteers, who didn't fight). The ratio is 1 chariot : 1 elephant : 3 cavalry : 5 infantry soldiers. In each of these large number groups (65,610, etc.), the digits add up to 18.
It is mentioned in the Mahabharata that, in the Kurukshetra War, the Pandava Army consisted of 7 akshauhinis (1,530,900 warriors), and Kaurava Army consisted of 11 akshauhinis (2,405,700 warriors). Thus, the total count of warriors who participated in the war, being 3,936,600 .
Kaurava Army and their Allies.
Kuru Army is a coalition of 11 Akshauhinis formed by the kingdom of Hastinapura in alliance with races like the Samsaptakas, Trigartas, the Narayana army, the Sindhu army and Shalya of Madra.
Pandava Army and their Allies.
Pandava Army is a coalition of 7 Akshauhinis, primarily the Panchala and Matsya forces, the Rakshasa forces of Bhima's son, and Vrishni -Yadava heroes.
The 4 types of units that make up an Akshauhini can also be seen in Chaturanga, the predecessor of chess.
References.
3. Akshauhini Wars in Detail - A Study