Question

is there a variable in the number 12? Is there one to represent the number?

Answer 1

is there a variable to represent it

Answer 2

no, the number 12 is a constant, meaning its value never changes. A variable is used to denotes values which can change depending on the situation.
In 2x =12, x takes the value 6. But if you plug x in a different equation, say 3x = 12, x becomes 4.
in x^2 = 25, the equation can be solved by two values of the variable x, 5 and -5.