Number of `Na^(+)` and `Cl^(-)` ions associated with each a unit cell of `NaCl` is:

Explanation

`NaCl` has `fcc` structures `8` at the corners is shared by `8` unit cells. Each of `6 Na^(+)` ions at `fcc` is shared by `2` unit shell.
`:.` No. of `Na^(+)` in a each unit cell `=8/8+6/2=4`
`12 Cl^(-)` at the middle of each edge of cell is shared by `4` cell and `Cl^(-)` at the centre is wholly belong to it.
`:.` No. of `Cl^(-)` in unit cell `=12/4+1=4`