How much money does someone need to retire?

Before you can answer that, you need to know how much income you will need in retirement, (and for how many years). So, figure out a retirement "budget" first.

Then you look at all your guaranteed income sources (pension, social security, other) and deduct their total from the budget amount you figured you'd need.

Last, you take what is left after deducting the other income streams from your target budget amount, and divide it by 4.5%. The answer to that calculation is how much cash you will have needed to accumulate by retirement.