I just spent the last few months working with a FA (financial advisor) to ensure my self-directed investments and retirement strategy were on track. (Yes, thankfully, as I expect to retire next year.) . Apologies to DL'ers abroad as much of this applies only to the US.
How much you need at retirement depends on a lot of things, as others have pointed out.
To develop a retirement plan there are two key points: 1.) understanding how much money you will require to cover all your living expenses in retirement and 2.) understanding where that money will come from. If you're lucky enough to get pension income, that and Social Security may cover the majority of your costs. Don't forget to factor in inflation.
Starting retirement with little or no debt is very important. Car loans and credit card debt should be paid off before you quit working. Mortgages are ok so long as you aren't upside down (owe more than the place is worth) and can safely cover the monthly payment.
If you live on $30,000 / year now and expect to deduce that to $20,000 / year in retirement, you need a sound, reasonable plan to achieve that reduction. I went through a couple years worth of credit card statements (found them online at the credit card company's website). Many credit card companies will give you annual summaries and sort the charges (groceries, drugstore, restaurants, etc.) That and my checking account statements really helped me figure out where my money was going currently and helped me understand what was reasonable to assume I could reduce in retirement.
The 4% withdraw rule assumes that, based on historical averages, your investments (typically 50% stock / 50% bonds) will return 4% annually or better, ensuring you never totally deplete the principal. The reason you want bonds and cash in your retirement portfolio is so you can tap into those during market downturns rather than selling stocks. You don't want to sell your stocks at a loss during a downturn or in a recession. Instead, your bonds and Money Market funds should maintain their value during those periods. Use those funds to meet your monthly expenses until the market rebounds.
Taxes - This is where I was confused and messed up my projections. Most people will owe federal tax on a portion of their Social Security payment since it depends on your overall earnings for the year; in some states, there is no state income tax on pension income and SS (hurray, I live in one of them); deferred income (money you put into a 401k or IRA pre-tax) may be taxed like ordinary income when you withdraw it (depends on a number of factors); your federal tax rate is based on your overall income, so your tax rate in retirement should be lower than it is pre-retirement. However that is not the case with everyone, especially when you reach 70.5 years of age and RMD's (required minimum withdraws of your retirement investments) begin. That can kick you into a higher tax bracket.
Don't forget to budget for healthcare in retirement. Medicare only covers a portion and you must pay for Medicare supplements. The cost of some supplements will increase based on your overall income in retirement (pension, SS, p/t work, etc.)
A couple of ideas for a safety net in retirement: A home equity loan, easier to secure while you are still working, can provide an emergency fund when needed; A reverse mortgage is sometimes an option, but don't get it until you need it and make sure it is with a reputable firm.
If you have a 401k plan at work, contribute enough to at least get the employer's match, typically 50% of your contribution, up to 6%. It's like free money. And don't borrow from your 401k. It impacts the overall compounded growth.
This stuff may not apply to everyone but I hope it's helpful for some.