How does this work with QFT? If an electron is just an excitation in a field then it being “one electron” is meaningless.

Yeah an electron is not a 'thing'. It's like saying there's only one wave in the ocean.

This could practically be a zen koan

It doesn't. It is theory for times proceeding QFT or modern quantum mechanics and nonsensical in its context.

It is like saying that meter is same everywhere. Electron is a quantized energy level of electron field, a measurement.