Tube Preamp: How many tubes does it take to .................

I think that sound reproduction fidelity is inversely proportional to the number of gain devices.  So do a lot of people.  Accordingly, I don't think it's coincidence that the best sounding preamp that I've ever heard uses only one-half of a double triode tube per channel as the entirety of the gain structure.  It's the Dodd Battery Powered Preamp.  A simply extraordinary amplification device.  No voltage rectification.  Huge signal to noise ratio.  Remarkably linear and remarkably transparent.  There's a reason Danny Ritchie still uses one to demonstrate his speakers.  To paraphrase Danny: "If I knew of something better I'd use it, but I don't."

My ARC SP9 MKll has one tube for the line stage part and one tube for the Phono section.

When it comes to transparency being perceived as a sonic trait from a Pre-Amp, I am very familiar with a model, that supplies this perception in a manner that is extremely impressive. The component responsible for this perception is the Korg Nutube

The Korg Nutube assuming the the role of a Triode in the circuit is something to be experienced.

I would suggest, if a line up of transparent Amp's were side by side compared, the model with a Korg Nutube as part of the circuit, will be a real contender for top place, as it has other very desirable sonic traits that work very well in conjunction with its transparency. 

A friend who is building Pre- Amp's using the Korg Nutube, has recently been at Hi End Munich 2023.

There was, in their and an accompanying individuals opinion, not any Pre's witnessed during demo's attended, that delivered transparency in the manner the Pre-Amp produced with a Korg Nutube can.

Tim de Paravicini's later (and still currently available) EAR-Yoshino pre-amps use far fewer tubes. The 868PL (phono and line) uses two tubes per stage, the 868L (line only) just a pair of 7DJ8's. The 912 uses the same two line stage tubes as the 868, with three tubes for phono gain.

The ARC SP-9 mentioned above by @curiousjim is a tube/FET hybrid design, not a pure tube one. That's why it uses only a single tube for each stage. IMO ARC hybrid pre-amps sound more like FET's than tubes (I've owned two).

My preamp uses one cathode-follower tube in the line stage but has 8 tubes (and 4 MOSFETs) in the power supply. My phono stage has 3 tubes for RIAA and 4 tubes in the power supply. The point is that tube preamps tend to use the most tubes regulating the power rather than amplifying the signal. And where there is a dearth of tubes, there is a reliance on solid stage regulation instead.