Too many variables to answer.
In a city? In the sticks?
Children? School Fees, Uniform, etc.
Car? Car Insurance, Fuel, Servicing, Road Tax, Tyres, etc.
"Luxuries" like True Visions, Broadband Internet, farang food and Air Con?
If no kids and living in Nakhon Nowhere, almost no luxuries, no car, and not going out to eat or drink, you'd probably survive. But is that the type of lifestyle you can live with?
On just 20,000 a month you'd struggle to get yearly visa extensions unless you were over 50 and had considerable savings to show in the bank.