One person tickling me in one spot is enough to make me lose control. Two people ganging up on me makes it harder to even try to defend myself, so I lose control even faster, but either way I wind up writhing helplessly and laughing hysterically. Three or four people guarantees my defeat but it's already not possible for me to laugh any harder than I already am.
I don't think I've ever had a team systematically target me, with one person, say, tickling under my arms while another goes after my feet, but I can only imagine that would be gilding the lily to some extent -- if my ticklish reactions are already going all the way up to 11 with the underarms person, then the foot person isn't going to be able to push it any further -- right? I'd be afraid to find out.