How much knowledge is necessary to have a good life?

The answer to your question depends upon what you want with your life, if you want to be someone who knows everything and be well known for it then the goal that you are chasing isn't knowledge it's fame and Creed.

But if you think knowledge is something that helps you form good relationship with your loved ones and function properly in the society then it doesn't take an Einstein to be living this sort of life, all you need is some basic human values that lie above everything else and your work towards those values and you will be all good with your life.