How many times can I extend my stay in Thailand on a US visa exempt entry?

You can only extend your stay once per visa exempt entry in Thailand. After that, to continue your stay, you would need to exit and re-enter Thailand (a process often referred to as border bouncing). However, if you've made frequent entries recently, or if you've been in Thailand for extended periods, this method might not be effective anymore. Also, there is a limit of two land border entries per calendar year.