@Giuseppe **********
if you're Embassy issues the affidavit of income letter you can use that and you don't have to transfer any money into the country.
If you're Embassy doesn't issue that letter you have to transfer the money in from an international account to a Thai bank account in your name only each month, every month for the previous 12 months before you apply for an extension.
That's how that works unless you're on a marriage extension and you legally work inside Thailand with the work permit, because then you can use salary to meet the financial requirements per month.