Amount of plant matter needed would vary by tank stocking levels, lighting provided, how much the animals are fed, etc. Pretty much any factor that would ditate maintenance coul be used to calculate how many plants are needed. I'm so super genious, so I don't know the fomulas for how many stems of a pothos are needed at a certain strength of lighting for a certain number of fish at a certain feeding rate in a certain sized tank, etc.
So far it seems that most just plant a few and let them grow like crazy until equilibrium is reached.