A 10 digit number? That is pretty extreme.

I would have guessed number 7, at first glance I saw a number from 1-10.
From my experiences with experimenting with telepathy, reading about it and discussing these ideas with other people is that the people who are most reluctant to consider the idea of telepathy as real are the ones that are most afraid to have their own thoughts revealed. People with nothing to hide are more open minded, needless to say.
Telepathy has been studied and the more two people know each other the better it works.
It's also easier to draw a picture that the other person is watching than guess a ten digit number for example.