For my first extension based on an OA visa, I got the 12 month statement, but immigration handed it back to me - not needed. Of course, I will need a 12-month statement for my next renewal to prove that I've met the money requirements all year.
I got LMG, 200k deductible, to meet OA requirements and most likely will never have any other use for it. Cigna inpatient is my key insurance, but not certified for OA requirements - but that might change later this year. This image shows rates from about July of last year.
A multiple entry OA allows you to enter Thailand an unlimited number of times as long as the original visa is valid, which is one year from the date issued. Each time you enter you get stamped in for one year, which allows you to stay for one year from the date of that entry.