The number of sodium chloride formula units present in a unit cell of $\mathrm{NaCl}$ are

NaCl is an ionic crystalline substance. 

 NaCl crystallises into a packing configuration known as face-centered cubic during the crystallisation process. 

The multi-molecule lattice structure of sodium chloride is composed of this packing configuration as a single unit.

The following properties are seen in face-centered cubic unit cells of this type:
 

• Atoms can be found on each of the cube's vertices in a face-centered cubic structure.
• The geometric restrictions, each of the vertices has $\frac{1}{8}$ of an atom.
  Similar constraints on geometry result in only $\frac{1}{2}$ of an atom being present on each face.
• Total number of atoms$=\frac{1}{8}(8 \mathrm{vertices})+\frac{1}{2}(6 \mathrm{faces})=1+3=4 \mathrm{atoms}$

As a result, each unit cell of a face-centered cubic structure has 4 atoms.

Hence, the number of $\mathrm{NaCl}$ units present in a unit cell of $\mathrm{NaCl}$ is 4.