I always wandered that myself, what is the realistic upper limit for number of ticklers?
I would say it depends on style of tickling. For fingers/nails I could imagine anywhere between 4 and 8 to be the upper limit, mostly depending on person tickled, and 6 being the optimal middle number in that estimate. 6 ticklers could nicely cover most weaknesses of a victim. 1 for each foot, 1 for legs, 1 for tummy and waist, 1 or 2 for armpits/arms/ribs. It is possible to add more, but not needed. If you can't break someone with 6 lers, I sure don't think so you could do it with 12, unless...
...feather irritating. I know some people are not sensitive to that and are having question mark overhead when seeing feathers mentioned for tickling. As someone who is most sensitive to this kind of tickling, I do have an idea how could it be.
For that kind of tickling, if focused, could host more lers strategically placed all over victim, but you need good conditions like nice bondage table. I could see something like 2 ticklers per feet, 1 for each leg, 2 for waist and belly, 1 for each armpit + parts around and 1 to focus on neck/face/ears. That is total of 11. Yeah, I can potentially see that work in reality if there is one gang leader who will make sure ticklers are really nicely positioned to not get in the way to each other. After all, ticklers are like at least 30 cm from each other and target completely different areas, except those on feet. They need to perform nice teamwork to nicely cover all weaknesses on each foot.
Would that be more effective than 5 ticklers? I have no idea. Never tried it, but I'd definitely like to do those kind of experiments. Hell yeah.