When mixing colors, is there a specific amount of each we should mix?

It all depends on the painting you are making. If you anticipate needing more, mix more the first time. In time, mixing colors and re-mixing colors will become so easy that it will become second nature. Some artists like to mix up small containers of colors and continually use these. I prefer to mix up new colors as I go. Sometimes, on the way to recreating a particular color I will discover a new one that I might prefer.