FIRE number is confusing to me…

For context: I am 24 y/o. How does someone my age come up with a FIRE number? All I know long term is that I don’t want to be in debt. How the heck do you figure out how much you’ll need to retire with? I don’t know the state of the world in 30-40 years, I don’t know the state of the economy either. Should I just not worry about this? I plan to continue to invest as much as possible into my 401k and Roth (currently at a 5%/8% divide for company match purposes (3%)) and continue to put additional