Here are some links to read re Becoming a Monk in Thailand...
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16-permanent-visa-for-a-buddhist-monk/
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69-can-a-foreigner-become-a-monk-in-thailand/
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4-what-are-the-current-visas-for-monks/
Ubon Joe had posted in 2014...
There has been no changes that I am aware of.
The visa would be a non immigrant R from an embassy or consulate.
For an extension of stay it would be clause 2.13 of immigration order
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2.13 In the case of studying Buddhism or practicing religious activities:
Each permission shall be granted for no more than one year. The alien:
(1) Must have been granted a non-immigrant visa (NON-IM).
(2) Must have been confirmed and requested by the National Office of Buddhism, or Office of the Prune Minister, or Mahachulalongkomrajavidyalaya University, or Mahamakut Buddhist University.
(3) Must have been confirmed by the abbot of the temple where the applicant is studying Buddhism or practicing religious activities.
Ubon Joe added...
A one year visa from an embassy or consulate would be a multiple entry visa that requires leaving every 90 days.
I think you are truly writing about an extension of stay (it is not a visa) you obtain at immigration.
You can get an extension of stay for studying Buddhism under clause 2.13 of the immigration order with the proper documents.
There is no 9 year limit that I know of. If you have supporting documents you can get extensions of stay for infinity.