Question: What do you call someone who plays multiple instruments?
A "polymath musician" or "musical polymath" might be appropriate terms for someone who plays several instruments. You'd have to be really good at each instrument (like, professional quality) for the term to be meaningful because most musicians play several instruments passably. "Musical polymath" usually means someone who does several different things in music like play an instrument and write symphonies.
a multi instrumentalist
When it comes to just instruments, then you would consider someone who plays two or more of them a "multi-instrumentalist".
When it comes to anything with music (instruments, composing/writing, recording, etc.), then the term "musical polymath" or "polymath musician" are used for someone taking part in two or more.
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