Do I need to show 20,000 baht in cash when arriving at Suvarnabhumi Airport with a tourist visa?

There are two times you're likely to be asked for proof of funds and they are handled completely differently.

One is when the are buying a visa at a thai consulate in another country that requires proof of funds.<- in this case you show a bank statement you CANNOT show cash

The other one is when you are stamping into the country and can be asked for proof of funds. <- In this case you show CASH (20K baht or the equivalent) you CANNOT show banked money or a credit card.

NOTE: there are no ATM machines air side <- before you stamp into the country.

Also I deleted the comments from people who said they were never asked for it. The fact that you were never asked doesn't mean NO ONE is asked for proof of funds AND failure to provide proof of funds if asked is grounds for denial of entry.

Good Luck