# What is the difference between "NI" and "Ni"? How many elements are in each?

Well, for one, $\text{NI}$ doesn't exist (on the other hand, ${\text{NI}}_{3}$ has been witnessed; ${\text{PF}}_{3}$ exists, so by implication of elements on the same column of the periodic table being similar in valence electronic behavior, ${\text{NF}}_{3}$ exists, and thus, so might ${\text{NI}}_{3}$).
Despite both of their ($\text{N}$, $\text{I}$) natural oxidation states being negative, the least you could have is charge magnitudes matching, which would require more than one $\text{I}$ atom bonded to $\text{N}$. If it did exist in this unrealistic exercise, it would be different in that it has two elements.
$\text{Ni}$ is the nickel atom, and clearly, there is only one element in one atom, because every atom derives from a single element (a neutral atom IS the element).