The Thai Customs department has a notice about this very topic on their website.
It says, in part
“For Foreign currency, any person who brings or takes and aggregate amount of foreign currency exceeding USD 20,000 or its equivalent out of or into the Kingdom of Thailand shall declare such amount of foreign currency to Customs. Failure to declare or make false declaration is a criminal offence.”
Here’s the customs website for specifics:
I do know that like most countries, the customs officers can’t and don’t screen every person, but if you were to be screened and you had more than US$20,000 (as noted above) you’d be in a very difficult position and subject to fines, seizure and other actions.