There are many sources for a 500,000 baht limit (in total value) for currency imports. I checked 3 embassy websites, they say the same.
***************************************************
Also, almost every country has police powers that enable cash or other valuables to be seized if they reasonably suspect involvement in criminal activity. This obviously concerns exporting from your country of origin (and transit airports) but I assume Thai police have similar powers. I regularly see police checks in airports in various countries for this and other potential issues. Declaration helps demonstrate innocent purposes.