Just one. It happened a couple times by two different women, one was a girl I dated, and she did it as as a favor because she knew I liked it. The other was a professional dominatrix and I had to pay her to do it. Neither was that great. I've yet to have it done by someone who's really into it, let alone more than one. But yeah, I have imagined like three. But aside from those, it was just playful tickling as a child.
I think it's easy to imagine multiple ticklers when you're not getting any at all. But if I could find just one to do it to me, who is really into it, that should do!