In the context of Mahabharata, eighteen is quite and important figure. It is no incidence that Duryodhana's army had 11 Akshauhinis (2,405,700 soldiers) while Yudhishthira had 7 Akshauhinis (1,530,900 soldiers). The chapters in Mahabharata are eighteen and these are called as Parvas and last, but not the least, This battle was fought for 18 days.
A detailed and interesting account of events of 18 days are available here.
A brief description of Mahabharata story will be interesting to read.