33NRR is the highest available with current technology. Iirc, you can get to an effective 35NRR by clamping a set of the highest rated muffs over top of the 33NRR plugs - it may be a point or two higher, but not much.

At that point your ear canals are close to being blocked, but the human body, mostly your skull, is still a good transmitter of vibration at high intensities and nothing you can put in the ear canal will address that. And NRR simply measures how much sound reaches your eardrum from the outside.

I suppose that putting a big isolation booth around your body while wearing muffs and plugs could get a theoretically higher number, but that's outside the definition of NRR. And if we're going off the board for high ratings, I think just moving a couple of miles away would probably be effective too