Does my passport need 6 months validity for a retirement visa extension in Thailand, and when can I apply for a new one?

Your extension will only be issued up to the validity of the passport so, in your case 6 months, then you apply for a new 12 month extension for your new passport, you lose 6 months.

Your passport doesn't have to have any minimum amount of time left to apply for an extension (not a renewal). But your extension can never go past your passport expiration date. So if you apply for an extension when you only have 6 months on your passport, you'll only get an extension for 6 months.

Are you asking about applying for a new passport? You can do that whenever you want. That's between you and your government, has nothing to do with this forum or with Thai immigration.

For your extension, you can apply when you have 30 days or less left at most offices, and some offices allow 45 days or less left.