The Falcon 9 has two engine-out capability.

I believe Falcon 9 has 2 engine out capability.

probably not any 2 engines though, right? I'd guess if the two engines on one side fail it can't recover, since it could only proceed with 1/6th capacity through the middle engines. (They have 6, right?)

I'm not familiar with the specifics of Falcon9 redundancy. But in theory, losing many engines can be completely recoverable, because all 9 engines share a single fuel tank.

Outside of atmosphere, a rocket's final speed and orbit depend on three factors: the mass of the payload, the mass of the fuel, and the inherent engine design efficiency. Notably, it does not depend on thrust. Burning one engine for 9 seconds results in a speed boost equal to burning 9 engines for 1 second.

Even if an engine fails and another one is set to reduced thrust, the fuel is still there, and can be burned by remaining engines.

Of course, this is a huge simplification, and loss of thrust while still in atmosphere or near the ground might be a problem.

I think thrust counts to a degree, as you'll lose velocity from gravity drag if the time to orbit is too high. The degenerate case would be if your thrust is less than the weight of your rocket (i.e. not moving).

Any 2 engines.

Side engine loss is compensated by its opposite side engine reducing thrust (along with those in-between - in ratio).

This leads to a longer burn and slightly greater fuel waste - but it is stable and was designed just for this very occasion.

They have 9, thus Falcon 9.