Everybody's numbers are different. Some of us spend more than others, some have expensive hobbies (like travel) and some have family to support.
First Calculation: I started by keeping a budget added up everything I spent every month and I kept doing that for a few years. I then adjusted for changes to expenses I expected to make once I retired - more travelling, fewer lunches in Central, having to pay for my own medical and dental etc. I then arbitrarily added 20% to the resulting number - I was working on the theory that I would rather add a few years to my working career now while I am in "peak earnings mode" that have to scramble for a job or cut back on lifestyle when I'm in my sixties or seventies. I also wanted a margin of safety which would both cover emergencies and allow me to reinvest some surplus income to offset inflation. This tells me how much income I need.
Second calculation: make an assumption on how long my money would have to last. I FIRE'd at 47 and DW is a bit younger than me, so I assumed it might have to last 50 years which, from a financial perspective, might as well be forever. Practically, speaking this means that I have to live off the income/profits generated from my investments rather than progressively drawing down on my savings which would be an option for a shorter expected retirement period.
Third calculation: decide on an asset allocation (stocks, bonds, cash, real estate, bullion etc) that suits your risk tolerance - over the very long time horizon I was hoping for, I was more concerned about the effects of inflation than year to year market fluctuations, so I went with mostly real estate and equities with enough cash to ride out any disruption. There are de minimus allocations to a few other things as well. I also kept some mortgage debt - interest rates were below yields on equities so there is a carry + it provides something of a hedge agains inflation.
Fourth calculation: take the expected return on your asset allocation and divide it into your budgeted annual expenses - the result is how much you need invested to retire (not including your home and any other non-icome producing assets).
This will result in a larger number than a lot of retirement calculators or financial advisers will come up with but it also results in a portfolio robust enough to handle a reasonable degree of adversity.