There is no "one size fits all" when it comes to answering the question: "how much money will you need to comfortably retire on"? It is entirely dependent on your retirement goals, desired lifestyle and how flexible you are.
The most common wisdom that you will find on the internet will tell you that the number is 70% of your pre-retirement income. That number will of course vary depending on your expenses which in turn will depend on where you are living. The good news is that - if you are flexible - there are different retirement options for almost every budget if you are willing to move where the costs can accomodate your income. For example an article in MoneyWise covers 20 different countries where you can retire on a lot less than you may think is possible.
Countries such as Portugal, Uruguay, France, Costa Rica, Portugal, Thailand are covered in the article where yes, appartment rentals can be found ranging from $350 to $750/month. If you have a retirement income of $2500-$3000/month for example, you may find that one of these countries could serve up a workable and enjoyable retirement.
If you are planning on retiring in California, New York or any major metropolitan area across the globe it would be very challenging to retire there on such a budget, unless you were sharing an appartment or a house and even then, it would likely still be challenging.
The other golden rule you will often read about in the media is the 4% rule. Whatever your final nest egg amounts to, you should only draw down a maximum of 4% of the value of that sum each year. So, for example, if you have saved $1,000,000 you would draw down no more than $40,000 each year. You may need to adjust that percentage up or down depending on the age you retire at.
If you add up your expected social security benefits, monthly income generating assets, dividends and the sum total of your anticipated nest egg you can arrive at a very rough number of what your annual and monthly income is likely to be. For example if you anticipate having a $500,000 nest egg by the time you retire that could yield a conservative 2% per annum plus $1900/month in social security benefits, you could anticipate having an income of $1900 + $850 = $2750/month without drawing down on your principal. If you draw down on your principal at 2% per annum you would have an approximate income of $3,550/month which would likely decline each year, as the annual interest would be less as you draw down more of your principal every year. We are not factoring in portfolio growth or losses in these numbers, nor inflation which are all potential.
Furthermore, whether you anticipate owning your own residence outright or still have a mortgage on it (or rent) will have a big impact on your disposable income and whether or not you can afford to keep living where you are or whether you may need to down-size or move altogether or add the sale value of your property to your nest egg.
As you can see from the very simplistic scenarios we have presented in this article, there is no one size fits all "retirement plan". We have also not touched on healthcare, assisted living as well as a number of other requirements that need to be considered as part of your retirement evaluation. As long as you are flexible, there is probably a scenario that can allow you to find a way to live reasonably comfortably in retirement. You may or may not realize all your financial goals, so It's always good to have a Plan A, B and C for a few scenarios. The optimal outcome is that your retirement goals come to fruition. However, just in case that does not happen, you will have greater peace of mind knowing that you have a plan for several other scenarios playing out.