The total number of anti bonding molecular orbitals, formed from 2s...

The atomic orbitals combine to form a new set of equivalent orbitals known as hybrid orbitals. Unlike pure orbitals, hybrid orbitals are used in bond formation. The phenomenon is known as hybridisation, which can be defined as the process of intermixing of the orbitals of slightly different energies to redistribute their energies, resulting in the formation of a new set of orbitals of equivalent energies and shape.

The number of hybrid orbitals is equal to the number of atomic orbitals that get hybridised.

Antibonding molecular orbital from $2s = 1$ that is $\sigma * 2s$

Antibonding molecular orbital from $2p = 3$ that is $\sigma * 2{p}_{x}, \pi * 2{p}_{y}and\pi * 2{p}_{z}$.

Total $= 4$