Check out something like FIRE calc: https://firecalc.com/
What you need to do is figure out the burn rate to sustain your family, figure out your safe withdrawal rate (e.g. 3.5%), then you can easily get amount of capital required.
A Mormon friend of mine, with 5 kids, spends less on food for his family than we do (family of 4) --he/his wife are just much better at spending on it. There are also factors like, it's possible to get a 3-4K sqft house for a very reasonable price & low taxes, vs. living in a tiny box in San Fransisco for 2x more.