It ranges from 20,000 to 30,000 baht but is also controlled by your card's daily withdrawal limit. The cartel of banks in Thailand charges 220 per withdrawal. You can go to the counter and withdraw money, but only at larger branches.
That's a good tip but what did you use for proof of address? I can assure you that each branch does not set its own rules but staff training is almost non-existent so each branch will behave differently.
Just keep trying different branches. They have changed so many rules the staff don't understand them. The UK embassy will not legalise documents but I don't know about the US Embassy