I think the question is more when and how often do you find it erotic. I'm sure there are instances for everyone where tickling is a little, some, alot, or not erotic at all. It is very much a case by case basis.
What you're asking is on a consistant basis, which would probably be around 21-30% in my opinion, and those people, of course, represent tickling fetishists who are active. The other 70% or so could find it erotic at certain times depending on the situation/level of arousal, but my guess is that it fluctuates.
EDIT:
As for tickling altogether, regardless of the person or of any fetish, I'd say 50%. While the tickling itself may not be erotic, it can lead to erotic situations in which it would be used further. I'd say theres a 50% chance of that happening with two consenting adults who are dating eachother, are married, drunk or high (unfortunately), and/or are at a party or event where physical activity and touching is more likely, etc.
Anything other than this where ticking would apply is N/A to the 50%. Obviously though, it can be said that the percentage would be much lower, the occurances less likely, and the eroticism involved less intense, implied or not at all.
Sometimes a person might think the other is just being cute, while the tickler means for it to be erotic, etc. Theres too many factors involved to come up with a conclusive percentage that is consistant with the whim of tickling. Its ever-changing and evolving, and its often determined by chance and circumstance perhaps more so sometimes than it is how the person(s) feels about tickling.
A fetishist won't always get a chance to take it to the level they may want (even if the person would be willing), and likewise, a "vanilla" whose interested in experimenting/being experimented on, may get more than they bargained for (which is often unfortunately which leads to people being put off by tickling and considering it a hang-up or personal phobia, or worse yet looking down on ticklers with contempt).
