Tricky question, but not for the obvious reason.
Vox is an intrument.
Therefore, Zep was a four piece. But wait! JPJ also played keys, mandolin, and guitar. Robert Plant played Harmonica. I know, but yes, Harmonica is an instrument. Trouble is, with these guys, they only did one thing at a time.
Okay, let's take a better example: Rush - three musicians, but the guitarist and bassist play keys either with their feet or fingers, and the drummer does the same with sticks. While they play the traditional guitar, bass, and drums. Three guys, lotta instuments.
In traditional combos, each musician does one thing. In modern combos, the options are quite a bit more open. I think the word "piece" doesn't work like it used to for modern music. So my definition rests on number of musicians on stage.
No, not dancers, thank you. Musicians.