It is really hard to say. I've only known, or talked to, a few females in my life who were absolutely not ticklish anywhere. Some girls might only be a little ticklish in one or two spots, but, they still had some ticklish reactions nonetheless.
I would say that the percentage of people who are absolutely not ticklish at all is low, maybe 5%. I've also heard, (and I am not saying this to be sexist, girls), that it is more likely for a higher percentage of males not to be ticklish at all, as opposed to females.
Just some points I've experienced and heard.
Mitch