Yes, I would say one for every day you're there. You may not need them all, but good to be able to just pitch them on your way out. Or put them in a bag once you get to the car.
We were there 4 days, and had needed them 3 days. One thing about the Dollar Tree ponchos - they stick to you. My dd really didn't like that at all. But it was WELL worth it as we wore them while waiting to see Illuminations - it was POURING down rain and we were SO glad we stuck it out. I really wish I would have gotten a picture - we were pathetic, but glad we saw it.
Hope that helps!
MomlovesDad