There is Data about Fetish
Some estimate say that 14 to 20 % of people would have A fetish, but I am quite sure that all these numbers are occidental centric (Foot fetish is rarer in Asia because they see it every day).
Also it depends on what counts as being a fetish, and people disagree on these things
Anyhow, a Bolognese study that is now quite old (2007) argues that :
Body part or feature is 26 % of fetish discussion group members, of which feet and toes at 47 %
This would mean that 1,71 % to 2,44 % of people have a foot fetish.
Tickling itself is not in any list because we dont have complete data, we just know that "Behaviors" is the second most category of fetish, and it is 18 %.
If I compare 2 specific terms that are usually typed on Google to find content (What you would type if you want to see some of it), I notice that Foot Worship is typed 5,2 more times than Tickle Feet
So we can never know for sure but this would make an estimate that :
0,33 to 0,47 % of people have the tickling Fetish
Sounds plausible to me, but unless someone decides to make a very big study by asking 10 000 random people (We need a large sample if we have to dig under the 1% rarest), we will never know
