2 vs 8 tubes in preamp, sound difference?

Depends what the tubes are doing.  Parallel triodes, SRPP or cascode configuration,  or voltage regulation or constant current sources. It's likely more will sound better but not definite.

While we often crave one dimensional solutions to simplify our lives, it's not possible to land at any conclusion on SQ based on the number of tubes in a preamp.  It's all about the implementation.

Why would more of any component be presumed to sound better?

It’s all in the implementation like 3 said. All of my components have 8 or more. I attribute the sound to the Audio Research design.