How many months of bank statements are required for extending a retirement visa with 800,000 Baht in Thailand?

It is not bank statements you need but your bank book. It has to be updated for the past 12 months including up to the day you apply for your renewal. Plus a letter from you bank which is valid for 3 working days after the bank gives you the letter. The 800,000 balance has to be in your bank 2 months before and 3 months after you apply for renewal. Minimum balance can not go below 400,000.

...so how does it work if you go through an agency and they deposit 0 baht on your behalf, presumably only for a very short time, to get visa extension. You will have no bank statement showing money deposited over a period of time...does anyone ever check it...or are you then stuck with an agency forever....just interested !!

You first need bank statements showing the 800k baht has been in your account for at least two months. Then for next year's extension you will need 12 months of statements showing the 800k baht has been in your account for three months after you received your first yearly extension, then to show you've had at least 400k baht in your account for every month for the rest of the year, and that you brought the balance back up to 800k baht at least two months before you applied for your next yearly extension.

If it's your first extension you should only need 3 months. Subsequent extensions need the full 12 months

1 years statement

12 months, you need to order from banks HQ, takes 7 days.

12. They need to check you kept the minimum amounts in over a year period.