T-10 as already stated. Why Ruger hasn't got on the bandwagon with everyone else and gone to T-15s I have no idea. The T-10 wrench they send with the rings is worthless (twists or breaks) and bits from Brownells, et al, aren't much better.
If you are going through that many wrenches I would guess you are overtorqing. 15-20 inch/pounds is plenty. Should be able to get that turning the short end of the wrench by hand.