Under the Treaty of Amity between Thailand and the USA, American businesses enjoy the benefit of operating their branch office in the kingdom or owning the majority shares in a Thai company to do most businesses that Thais can do. What about work permits for American businesses in Thailand? We have the answer for you.
If the majority shares of your US company is owned by American citizens with more American officers/directors than other nationalities, you can apply for a Foreign Business Certificate to operate a branch office here. Even though you are covered under the Treaty of Amity and Economic Relations between the Kingdom of Thailand and the USA to be able to do the businesses like most Thais can, you need work permits for all your expat staff. Operating as a branch office, you will need at least 1 Thai employee for each work permit you sponsor. There is no ratio of capital per work permit.
Alternatively, you can set up a new company under Thai laws with majority of the shares owned by Americans. (Read here for the process to register a Thai limited company.) If it is set up as a company, to support each work permit, it needs THB 2M registered and paid-up capital and 4 Thai employees. Note that shareholders do not need work permits, but foreign directors do.