The "number" is always part of a big debate. There's no right or wrong.
Usually, they say that you can maintain your wealth (adjusted for inflation) indefinitely by using the so-called "safe withdrawal rate" [0], which people put between 1% and 4%.
So, say that you have $1M in wealth, and you pick your SWR at 2%. It means that you can use 2% of that, or $20,000, every year, knowing that your wealth will keep growing at least by the inflation rate, for a long time (30 years, or 100, or whatever).
If you have $10M, you can spend $200,000/year.
Clearly, it depends on your lifestyle how much you need to have saved in order to FIRE (Financially Independent, Retired Early).
All of this assumes that for the next 30, 40 years, we will not see any catastrophic or monumental changes in how the financial system works.