Most places I looked advised 1 litre would do 10-12 handfuls. We've got about 90 guest coming, and have ordered 5 litres - as I would like a good confetti shot. I'm putting them in small chiffon bags. Probably do everyone on the front row/aisle, then every other. I've also ordered another litre to go in the baskets of my flowergirls. Any extra from the bags I'll just chuck in there.
I would definitely go with what they've suggested, as that seems to be the norm from what I've seen. For clarity, look at a jug to see how big a litre is - I have no idea how someone got 50 cones from 2 litres!
Hope this helps! X