If one is enough for most of us then I think that we're hoping for these prices of Bitcoin to shoot for the next years to come. We don't know when but the time will come for these prices.
$1M
$5M
$10M
So with these prices, 1 BTC is enough for retirement.
I think the main point that has not been talked a lot is that each country and city, as well as lifestyles are going to change this number dramatically.
One person might be living happily with 1BTC, whereas another person might struggle with 10BTC.
It really depends on many factors, not only including the volatility of the price of BTC itself.
I don't think there's just one answer to this.
You're right. There is no bad answer to this, we're all living in places where we know the cost of living.
But someone from those expensive countries have also an idea if they should migrate elsewhere and enjoy their retirement with how much BTC they've got.
It all lies with availability of their plans and if it's in line with what they've planned of going to places that are ideal for them.
Someone can live off with 1 BTC in the future if it hits $500k to $1M as a retirement money but I wonder how much inflation have risen by that time.