Hi Hannah! Lots of good input on this one. I, personally, think of the spiral as a "figure" rather than a dance by itself. Admittedly it takes a while to run the figure depending on the size of the crowd, but yeah, like someone brought up using the spiral as part of a Grand March and I use it there too. I mostly use it near the end of a circle dance like La Bastringue. So after a while, when circling left I'll say, "OK I'm coming IN. keep circling left. New rules: follow the leader and don't let go".  Maybe the band changes the tune or not.. Then maybe a few reverses of direction, maybe a few duck under arch (Grapevine Twist-esque). Then wind it up and unwind it, get back in a circle and "one more time" and back to the original tune and dance. 

the trick is definitely when you get to the center of the spiral making the dancers think you have miscalculated , muttering "I'm SURE this worked the last time I tried it".. Then when you unwind it,.. ahhhhhh the crowd goes wild  hah hah

I would say the more the merrier. This figure probably works best with more than 30 dancers.. Less than that and there's not nearly as much drama. If the crowd gets too big, of course, the speed of the dancers is faster and faster as you get near the outside of the spiral so  be aware and adjust speed. 

have fun!!  - bill