If I leave Thailand and wanna be back again, How many day (at least) do I need to spend outside Thailand before coming back? And How can I apply COE outside my hometown?
TLDR : Answer Summary
You can return to Thailand without a minimum number of days required outside the country. To apply for a Certificate of Entry (CoE) while abroad, you should contact your local Thai embassy, as most embassies issue the CoE regardless of nationality, although visas must typically be obtained from the embassy corresponding to your passport.