All formulas have how many variables

All formulas will have more than one variable. You need at least one independent variable and one dependent variable, so a minimum of 2. 

For example, the formula for the diameter given the radius is: 
d = 2r 

The formula for area of a square is: 
A = s² 

The formula for the area of a circle is: 
A = πr² 

And you can have more, like the volume of a box is: 
V = lwh 

Or the amount of money you have with periodic compounding is: 
A = P(1 + r/n)^(nt) 

 
More than 1