What Are the Two Free Parameters in Nondimensionalization?

[INeedHelpTY]

Please check the image, I got it from wikipedia which is similar to the problem i am trying to solve, just a quick question, there's a line on it that says "Since there are two free parameters, at most only two coefficients can be made to equal unity.", i want to know what are the two free parameters, and how do they determine it? This is something that's mentioned in my textbook as well which I am confused of. Thanks

 

[fzero]

The free parameters are $$x_c, t_c$$ that were introduced by defining

$$x = x_c \chi ,~ t = t_c \tau.$$

The idea is that you can simplify the form of the equation by choosing $$x_c, t_c$$ in terms of $$a,b,c,A$$.