No its not 20k baht is the amount tourists need, in theory. The Thai customs site says $20k undeclared, but forms says $15k TIT
https://wise.com/gb/blog/taking-cash-in-or-out-of-thailand
Details of the customs regulations in Thailand can be found on the Thai Customs Board website.
According to the details available online, you can bring up to $20,000 - or the equivalent - into Thailand, without needing to take any specific action. Carry in excess of that, and you have to complete a declaration when you arrive - more on that later.
However, it’s worth noting that the actual declaration form you're required to complete states that amounts worth over $15,000 should be declared.