I also agree that it depends on the individual. From my personal experiences, though, I tend to get more hysterical if more than one tickler attacks me. I know I can fend one off, but once I figure that my defenses are overwhelmed, I lose totally lose my ****. The max I experienced was about six, and I don't think I have ever giggled so hysterically in my life. The ticklers, my friends who opted to give me a taste of my own medicine, said I sounded like a bleating lamb xD
So, I suppose one tickler, per tickle spot, wouldn't be unreasonable...? 😛