The number of bitcoins will depend on the kind of life style you would want to live...you can consider using the bitcoins to set up your own company(s) maybe set up a family farm....The options are endless 
otherwise between 100-10000 bitcoins things should be okay for a retired bitcoiner
Lol I think 10k bitcoin is the amount that is too large. I'm sure you will not reach that amount even if you use a variety of methods to be used within a period of 20 years. unless you're running a large business then it is a popular business and explode. you can become a millionaire from itotherwise between 100-10000 bitcoins things should be okay for a retired bitcoiner