The Latin letter n is commonly used in math as a variable.
Related
Latin Capital Letter N | Symbol
The capital Latin letter N is used in mathematics to represent the set of natural numbers. Usually, the letter is presented with a "double-struck" typeface to indicate that it is the set of natural numbers. Otherwise, N is also used as a variable.
Links
Variable Definition
A variable is a symbol that represents an unknown or indeterminant quantity. Variables can represent numbers, vectors, or other mathematical objects, depending on the context.
Latin Capital Letter N | Symbol 
Variable Definition