So where should the tubes be?

Just lurking and learning. Why do the number of tubes in a component vary so greatly? 

#of tubes? Depends on what the amp is doing and how it’s configured. A power amp could have a single power tube (SET) or it could have 8 paralleled push-pull output tubes. A phono amp could have only 2 tubes for gain and equalization or it could have an output buffer, tube regulators and rectifiers etc. You can see the 11 tubes in my phono amp here

It depends on a lot of design considerations.  If the component has AC from the power outlet converted to DC by a tube rectifier, that is at least one tube.  Some gear also use tubes to regulate voltage.  At earlier stages of amplification, most, but not all, use tubes that are actually two tubes in one glass envelope doing two functions.  Different circuit designs will require different amounts of tubes even at early stages of amplification.

As for the output stage, both design type and amount of output power determine the number of tubes.  A single-ended amp can have as few as one output tube per channel.  Push pull amps will have at least two tubes per channel, and the specific amount of tubes will depend on the amount of power the amp is designed to supply and the amount of power delivered by the specific type of tube chosen (different tubes vary greatly on output, my push pull amp with four output tubes per channel delivers only 5 watts per channel, a different design might deliver 100 watts).  Then there are output transformer less amps that typically have many output tubes because they utilize many tubes in parallel to instead of a transformer to deliver the power in the form that can be used by the speaker.

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Why do the number of tubes in a component vary so greatly?

function of the item and its design

More tubes = better sound for me. Components with tubes:
CD player, Preamp and its separate power supply (tube rectified), Monoblock Amps. Every component except my speakers and subwoofers.