Evidence of three months of a £10,000 UK bank balance is what you need to show in the Royal Thai Embassy London eVisa application for a 90-day, single-entry, extendable retirement VISA.
Evidence of monthly income of not less than 65,000 baht (approx. £1,625) or having the current balance of 800,000 baht (approx. £20,000), e.g. bank statements, proof of earnings, for at least 1 month is what you need to show in the Royal Thai Embassy London eVisa application for a 1-year, multiple-entry, extendable retirement VISA.
Two months of 800,000 baht Thai bank balance is what you need to show at your local Immigration Office if applying for a 1-year Retirement EXTENSION when 'converting' from your visa or visa-exempt entry.
Depending on where in Thailand you hang your hat, 'visa agents' charge between 15,000 and 25,000 baht for the LATTER with or without the financials (but trend towards the higher price for those who cannot self-fund).
https://london.thaiembassy.org/en/publicservice/84508-non-immigrant-visas?page=5d6636cd15e39c3bd00072dd&menu=5d6636ce15e39c3bd000733c