While researching the world of vacuum tube audio amps, I noticed that many commercially available stereo amps have an odd number of tubes. Here's a random example.
How can a decoupled stereo sound be produced using a single tube?
We utilize PSVANE/Shuguang brand KT88 tubes for vigorous and forceful sound (PSVANE/Shuguang brand randomly distribute). The 6N8P tubes provide a soft and dynamic sound stage and the 5Z3P (This tube is singular)tube acts as the rectifier.
2 Tubes are "Pre-Amplifier tubes (1 for left speaker and one for right speaker in a stereophonic system)
2 Tubes take the Signal that has been amplified from the Signal Source by the Pr-Amplified Tube into a Signal that can be amplified by a power amplifier because a power amplifier requires a minimum amount of voltage peak to peak in hthe signal it's amplifiying and the pre amplifier provides the usually weak signal from the orignal sound source into a Voltage level large enough to feed the Power amplifier which provides enough power to power and 8-ohm Speaker. The KT99 are the two Power Amplifier tubes and according to this website:(https://www.thetubestore.com/power-tubes/el84-6bq5-tube-types)
The last tube, the 5Z3P as the amazon.com description states, indicates it is for rectification which uses only one tube in amplifiers unl;ike the full bridge rectifier for alternating current to DC rectification because AC/DC rectification only happens at a frequency of 50-60-Hz whereas audio is, conceivably, rectifying up to around 20-kHz so full bridge rectifiers would create a 40-kHz Signal to be pre-amplified and then power-amplified along it's path from signal source through the amp and eventually through the voice coil and cone of the speaker being powered.
The description of the amplifier in your example clearly states that one of the five tubes is the rectifier. It should also be pointed out that many vacuum tubes contain more than one function, e.g. dual triodes (12AX7, etc.), so that a stereo preamp, at least, could be built from one tube.