I personally think that anyone who has assets worth 2c more than my assets is rich and has a substantial amount of money. But looking at it that way, everybody’s answer would be different.
It seems to me that if we want to be objective, not personal, the answer is
F.I.R.E. ——> Enough to be Financially Independent and Retire Early.
And that (if you want a specific amount) depends upon one’s personal comfort zone and where one lives.
Once upon a time I was looking for the perfect place to live. For around 50 years I moved around… At first it was “bargain paradises” where a small net worth could pay for comfortable living — Like today, $100,000 would seem like a fortune in a “socialist paradise” like Cuba where people earn and live on $30 a month. India was also cheap—if you lived like a local. But for me, after living in such places, I determined that towns where rich people live, like Aspen Colorado or Monaco were much more pleasant. If you had the resources, high rent districts were better than being the richest person in a 3rd world neighborhood.
Thus, another answer to the question might be: “A substantial amount of money is enough to live comfortably in a place where rich folks live.”