Do I need savings in addition to proof of monthly income for a retirement visa in Thailand?

YOUR STILL GONNA PUT FUNDS IN THESE BANKS..WOW

You need a thai bank account with required amount in it and it must show its been there 3 months your bank book is not proff to imigration you need and approved bank statement issued from the bank

if you are a U.S. citizen, you can apply for the 90-days Non-Imm-O visa in your home country using monthly income. But for the application to the extension of the 90 days stay permit to a 1-year stay permit based on retirement, you will need a minimum of 800,000 THB in a Thai bank account in your sole name, seasoned for 2 months, at least for the first year extension of the stay permit. Because your embassy in Thailand will not issue an affidavit of income for Thai Immigration . . . .only for the second year, after having collected 12 monthly transfers of a minimum of 65,000 THB with proof they have been sent from abroad, month for month, confirmed by a 12-months Thai bank statement, Immigration is willing to issue the 1-year Extension of Stay Permit using monthly income

You need 400k

When applying for a 90-day Non-O visa for retirement based on monthly income from outside Thailand—which is generally recommended—you are required to provide proof of income for the past three months exceeding the minimum required amount. Acceptable documentation includes the last three pay slips, a three months statement from your pension provider, or the last three months’ bank statements.

Which visa? What's your nationality? Can't get accurate answer without that info.

the monthly amount is enough, I do like this.

If your Embassy verifies your income then you can use the monthly financials. Otherwise you can switch to monthly financials only from your 2nd extension of stay. For the initial Non-O and 1st extension you will need banked funds.