you can get a 30 day extension, THB 1,900.- at the local Thai Immigration on Koh Samui on your 30 day entry stamp called visa waiver or visa exempt). Thanks God that you included your nationality. Enjoy.
every entry can be extended once for 30days, so it worked like the old triple entry visa. A Multiple Entry Tourist Visa allow unlimited entries within the 6 month validity period. Each entry is good for 60 days and each entry is extendable by 30 days. After a 90 day period is up you can join a border bounce to activate another 60 day. If you enter before the ENTER BEFORE date you get another 60 days do another 30 day extension , so you can stay up to 270 days, same as the triple entry before. In theory you can also get an addl 7days extension denied after the initial 30 days are completed, but you need to pay another THB1,900.-