Let us take an example of oxygen molecule.
Oxygen atom is having electron configuration 1s2²s2²p⁴. So the oxygen molecule is having total of 16 electrons.

Bond order indicates the strength of a bond and is defined as
Bond order = (no. of electrons in bonding orbitals )/2
                        – (no. of electrons in the antibonding orbitals )/2
For oxygen, considering only the valence electrons (2p orbital)
Bond order = (8-4)/2 = 2. So the result is a double bond.
Bond order =1 (single bond); Bond order = 2 (double bond) e.t.c