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.

You have to take the ceteris paribus approach. Would the same basic design be improved by adding more tubes? It depends on how the design is implemented. Say you have a two tube preamp and it sounds good. Then you add another two tube voltage regulator which tightens up the bass and soundstage. Now double it for stereo and separation improves. Now replace the anode resistor with a tube constant current source to improve the fine details. Replace the diodes for a tube rectifier may add better dimensionality etc...

But they also use MOSFET output buffers and solid state voltage regulation. Anyway, unless it has a phono stage, a preamp is a fairly simple device.

@emergingsoul Did the OP simply mean # of tubes or # of active elements? Just because an ARC SP9 has 2 tubes does not mean that it does not have any other active components. Then the argument is tubes vs solid state and not the number of tubes.

Like acting, gardening, and cooking, you either have it in your soul or you don't. 

So, tell me, how does one learn soo much about audiophiledom.