No exact numbers on this, but like most of the posters above the term "A lot" fits the bill.
Part of the reason is that paraphilia (what most call fetishes) tend to nest under each other.
A foot fetishist can express in a number of ways. The big split is visual vs interactive.
The visual foot fetishist is aroused simply by seeing a foot. The context of that foot is not all that significant.
The interactive foot fetishist wants to do to, or have done by, the foot. It can be one or more things.
AND a person can be both visual AND interactive (Unified)
So tickling, a fetish unto itself can easily fit under an interactive fetishists likes, and just seeing a foot beign tickled can be enough for some visual ones.
So you get crossover.
For a lot of ticklers, feet are the start and end of their interest in things to tickle. They probably would be better classed as Interactive Foot Fetishists with a specific preference, but fully qualify for the tickling tag also. And they might identify with either more or less based on the moment they are in.
The two paraphilia are forever linked by the fact that feet are ticklish, and thus both groups will have common ground. And many will like both sides.
Myriads