total number of molecular orbits is equal to the total number of atomic orbitals from combining atoms

The molecule H₂ is composed of two H atoms. Both H atoms have a 1s orbital, so when bonded together, there are therefore two molecular orbitals.  

Bonding molecular orbitals have less energythan the constituent atomic orbitals before bonding

Antibonding molecular orbitals have more energy than the constituent atomic orbitals before bonding.  

Bonding molecular orbitals help stabilize a system of atoms since less energy is associated with bonded atoms as opposed to a system of unbound atoms.

Likewise, antibonding molecular orbitals cause a system to be unstabilized since more energy is associated with bonded atoms than that of a system of unbound atoms.

Following both the Pauli exclusion principle and Hunds rule, electrons fill in orbitals of increasing energy.

Atomic orbitals are best formed when composed of atomic orbitals of like energies.

Molecular Orbital Configuration of Li₂:

(s_1s)²(s^*_1s)²(s_2s)²

The bonding (s_1s)² and antibonding (s^*_1s)² cancel each other out, leaving (s_2s)² as the valence electrons involved in the atoms bonding.