You will need to show several years of passive income (Private Pension, State Pension, Income from investments if you have it ie. Interest, dividends etc), and in 5 years time, be in a position to show this income is still there at the qualifying level - applicants have this 5-year check position assessed when the visa LTR-Visa application is submitted, and processed.
Thailand has a wide range of visa options to attract applicants with different circumstances, ambitions etc. In each and every case, there are terms and conditions which are generally clear, and if you meet them as an applicant, then visas are obtainable. A good agent, with a track record is also helpful to have as support, if necessary.