As far as I know its 400,000 THB in a Thai Bank in you're own name (not a joint account) or a monthly income.In the past it could be a combination, the money in the Bank should be in 2 months before or 3 months before depending where the information was given, I would go for 3 months to be on the safe side as things stand. Im off to the Immigration next month, so I will check that out. I read on the UK.GOV they advised at that time to have the money in a Thai Bank.
Please note each Immigration office will vary what they need to see IE my address is BKK, so I have to go by what they want in BMA, that could differ from Chiang Mai or Phuket for example.
However the monies needed will be the same.