How many bitcoins does a person need to retire, what is the cashout plan and why?
This depends on several factors affecting the wealth of a person.
Generally if a person lives in a third-world or developing country, 1 BTC is enough to sustain them for at least years. Given that the standard of living in those said countries is relatively low, such amount would equate to at least a significant value. Probably around 10 BTCs would be enough for them for retirement.
Though this may be the case for developing countries, people who lived in a developed country has a different value. Obviously, the standard and cost of living are high which means that more than 10 BTCs would require them for retirement.
At the end of the day, it really depends on the status of the person on how they would perceive "retirement" as it is purely a subjective question. For me personally and as someone who is living in a third world country, 10-20 BTCs would definitely suffice as a retirement fund.