One account only. The amount has to be 2 months before and 3 months after. Minimum balance can be 400,000 after 3 months. Each renewal you have to give photocopies of your bank book going back the previous 12 months. At Jomtien Immigration I had to bring in my updated bank book after 3 months of receiving my renewal.
For your first O retirement visa hire a Visa agent and let them do it for you after you arrive in Thailand. Sure you can do it yourself for 1900 BHT, which is a lot cheaper but an agency takes away all the hassle
For the first visa it can be better to go to an agency for the first one. It will cost you more but you will have less to complain about. Every year after do the renewal yourself.
When you renew your retirement visa you need a letter from your bank and also photocopies of your bank book. First page and all pages going back one year. Don’t forget to sign photocopies.
Never said you cannot do yourself. But it is so much easier for an agency to do for you the first time. Especially deal with the financial requirement.
For your first retirement visa go to an agency to get the visa. This will cost you more money but everything will be looked after. The following years do the renewals yourself. Or you can get the Elite Visa but this is very expensive.