Quite a big question to answer.

I don't think there is 1 fixed number for everybody. But my general rule of thumb is that if you have enough money to generate passive income which more than covers you expenses, then you are financially independent.

So to calculate that, you have to ask yourself how much are your expenses annually. Then you estimate how much returns you can reasonably make from your existing savings/investments.

Say if you spend $30,000 per year based on your current lifestyle and you have $800,000 which you are relatively sure that you can get a 5% ROI from, then you are financially independent. Because, a 5% ROI from $800,000 means you have $40,000 per year but you are only spending $30,000. You are still growing your wealth by $10,000 every year.