My suggestion is to do an experiment in which you vary the amount of enzyme at the desired substrate concentration, and measure product formation as a function of reaction time over the relevant period of time. Then you can choose the amount of enzyme needed to accomplish the reaction.
To define the ratio to be used you first have to set the time and the temperature of incubation. Usually one is looking for very low substrate/enzyme ratio to limit the level of contamination and low temperature to avoid heat induced protein denaturation and therefore the time of incubation has to be extended.