We are usually concerned with one reaction. That is, the production of one specific set of products from a specific set of reactants.
The number of values of c/d would be the number of possible ways that a and b could recombine to form different pairs of products c and d. (You might get different reactions at different temperatures, for example. Or, you might get different pars of ions.)
Usually, the number of values of c/d is one (1). (Of course, if you simply swap what you're calling "c" and "d", then you double that number, whatever it is.)