The Battle of Pharsalus is one of the most decisive battles of the Roman civil war. After their loss in the Battle of Dyrrhachium, Caesar's forces were battered and demoralized. However, Pompey did not sieze and initiative and took a passive approach to weaken Caesar's forces through delaying tactics.
The Roman senate however, wanted a decisive victory and forced Pompey to engage Caesar. With almost a two to one superiority in numbers according to some sources, Pompey was now ready to crush Caesar. However, Caesar's troops were battle hardened veterans and experienced in combat. Even though they knew that their forces were outnumbered, their morale was still high.
The forces had faith in Caesar's tactical brilliance and this would be proven in the battle. Pompey had a significant superiority in cavalry and he planned on using this to win the battle. Pompey would engage Caesar's infantry and use his cavalry to swing around Caesar's right flank and attack his troops from the rear.
This was exactly how Caesar had anticipated Pompey to act. So, instead of having conventional three lines of infantry, he had six cohorts from the third line deploy a forth line held in reserve behind his right wing. When Pompey attacked Caesar's weak cavalry, they expected an easy win but were surprised by the presence of reserve infantry from the fourth line.
Pompey's cavalry were routed and Caesar took the initiate to drive off the enemy to their camps. Regardless of the number of troops that faced Caesar, it is clear that he was significantly outnumbered and after the battle, Pompey's army was decimated and ceased to exist as an effective unit to challenge Caesar. The Battle of Pharsalus turned the tide of the civil war and paved the way for Caesar to become the dominant leader of Rome.