you can visit as many time as you want on visa exemption until the IO will suspect that you are living in the country without an appropriate visa and will deny your entry. There is no limit of exemption of stay stamps
technically it’s 1 calendar year, practically the IO at his own discretion can deny the entry if he suspects that you are living in the country without an appropriate visa and in the case that you mentioned the chances that the IO will think this way are high
it absolutely depends on the length of the stay, you can enter even 30 times on visa exemption in 1 year and have no problem, the OP intends to live permanently in Thailand with visa exemption through the years and this have high chances of trouble
question: if you have a multi re-entry permit on the visa and you proceed to 60 days extension it will be valid for the extension or need to purchase a new one ?