Matters relating to retiring to Thailand are entirely determined by personal circumstances and desires!
I have lived alone, albeit seven years ago, that I did for about six years around the Chiang Mai area, no commitments, no car, three small houses each self-contained with a garden in the countryside (B5000 for all three - really good for me! Not slumming it in the slightest!!
Can live on a monthly expenditure of a good deal less than B20,000 (closer to B15,000) per month and having everything I need and desire!
The question about "how much" money can be very low with considerations in my first sentence.
The point I am making is that some people want an inexpensive living and I have proven that it is possible as a single person.
So back to my first sentence, if we are talking more people, children, education, cars, etc, one doesn't have to manage or cater for as a single.
From a low baseline that is easily possible, the only way is UP in the form of more necessities and luxury (desires)
Yes, I know, personal circumstances might include Health insurance, live in Bangkok, Phuket and so on but I speak for what is possible for an everyday living without "what if' eventualities.
There are other factors such as home cooking, biking, local transport - sawngthows for example
None of this is part of my current circumstances - but I am retired in Thailand for 21 years, now living with my Thai wife in her very nice house.