Found this guideline article What Does Tax Residence Mean in Thailand?
Whether you incur a tax liability in Thailand depends on residence and source rules regarding taxable income.
You are considered a Thai tax resident by the tax authorities in Thailand if you are present in the country for a total of at least 180 days in a given tax year. Your assessable income is Thai-sourced income, as in income derived from sources in Thailand.
You are also subject to Thai taxation on foreign-sourced income, but only if this income derived from abroad is remitted to Thailand in the same year it is received.
Personal income tax must be filed by March 31. According to the Thai Revenue Code, individual taxpayers are classified into five categories, and assessable income is into eight categories.
When it comes to such income as employment income, all tax benefits are assessable unless expressly exempt by law. These include bonuses, bounties, directors’ fees, gratuities, house rent allowances, salaries, and wages
https://nomadcapitalist.com/global-citizen/establishing-thai-tax-residence/#:~:text=affordable living standards.-,What Does Tax Residence Mean in Thailand%3F,in a given tax year.