Enzyme concentration
The higher the enzyme concentration, the more enzymes there are to form enzyme-substrate complexes, leading to an increase in enzyme activity.
This happens up to a certain point. Enzyme activity then levels off (plateaus) as there are not enough substrate molecules to react with the extra enzymes.
Optimum
The The temperature, pH or enzyme concentration that allows the enzyme to work at its best. value for these factors is the value that allows the maximum rate of reaction.