Do I need a monthly income of THB 65,000 if I have THB 800,000 in a Thai bank account for a retirement visa?

One or the other not both

You bank 800K baht in a thai bank account in your name only (for 2 months before you get the year extension based on retirement)

OR

you show income from abroad of 65K baht a month via the letter your embassy gives you (affidavit of income from abroad notary document)

BUT

if you come from a country whose embassy here doesn't issue the letter you have to transfer in to your thai bank account the 65K baht a month, each month, every month for the previous 12 months before you apply for your yearly extension