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Can one person have multiple Ioun Stones circling their head at once? What about one in a Wayfinder and others around their head?
I just discovered the cheap cracked ones, and I could see picking up two or three different stones for the same character.
Since afaik they don't use up a slot yes you can have as many as you want.
Be warned that they scream "destroy me" (or possibly "steal me"). Buy some Wayfinders to keep them in if your GM is that kind of person.