I think the group can be any size, as long as at any time there is only one person doing the tickling. Except, as Skipa said, if two are really working together that's fine.
I think it is really frustrating, as a ler, to tickle a spot not knowing whether it tickles or not, because your lee laughs while being tickled at a different spot already. From a lee perspective, say I am tickled at one spot by someone, and I am really suffering but at the same time enjoying it, because it's just the right intensity, it doesn't hurt, etc. Now comes someone else and ruins the situation by wiggling his finger in a completely different style, maybe hurting, maybe not having an effect at all. What a bummer!
Two lers working together could for example take turns. One at my armpits, one at my thighs. Once they have understood what gets me going they might transition to tickling me at both spots at the same time.