Benzene has 6 molecular π orbitals. The bond order is half the difference between the number of bonding and antibonding electrons.
Of these, three are bonding and three are anti-bonding. The six π electrons go into the three bonding orbitals.
π BO = ½(B–A)=½(6–0)=3
This is the π bond order for 6 C−C bonds.
For one C−Cπ bond, BO = 3/6=0.5.
For a single C−C bond in benzene, the total BO = σ+π=1+0.5=1.5.