There are many ways to design pre-amps. And many different designs can sound good based on execution and taste.
The basic functions of a line stage pre-amp:
- Select from multiple sources
- Support maximum source input voltage (no overload)
- Control attenuation (most common) and amplification as needed
- Provide a high enough input impedance to the source to not affect the frequency response or dynamics
- Deliver enough gain to drive the amplifier to full output
- Provide a low enough output impedance for the connecting cable and amplifier to not affect the frequency response or dynamics
- Add minimum noise
- Add minimum harmonics (a.k.a. "distortion")
- Minimize phase and timing errors
- Do not restrict bandwidth
If you prefer a KISS approach, a single ended pre-amp can get by with one triode gain stage per channel. This could be one dual triode tube, or two single triode tubes. Tubes by nature have a high input impedance. If chosen carefully, a simple single triode stage can meet most of the above criteria usually for a short cable and highish impedance (often tube) amplifier.
There are many other approaches. A design like the classic Dynaco PAS uses feedback as so requires two high gain tubes stages (one tube) per channel. Some designs use a cathode follower which requires another one or two tubes. Other designs (like some Conrad Johnson) use many paralleled triodes per channel. If a pre-amp is fully balanced, then everything is doubled.
As mentioned previously, some pre-amps use tube rectifiers (one shared or one for each channel) and some designs also use tube voltage regulators.
As you add functions and features you can add even more triodes/tubes. A phono stage adds a minimum two triodes (one tube) per channel but can be more. Other features requiring more tubes can be a tape loop and phase inverter.
So, depending on performance, features, and convenience requirements/preferences, old and current pre-amps can use few or many tubes. This variety makes our hobby fun, IMO.