To add to pt's thought....
People generally know a lot of people. Think about your classmates, relatives, co-worker, x's, etc. that you have known in your life. In fact, how many people do you interact with on a regular basis? I personally have weekly interactions with well over 50 people right now.
The chance of having a particuar person you know be an INTJ is represented by a Bernoulli distribution which then means (assumng independence) that probability of k out of n people you know being a random variable is represented by a a Binomial Random Variable.
You would then just sum up k*prob(k out of n being INTJ) for all k from 0 to n to get the expected number of INTJs you know. Incidently, the result of that is the very common sense answer of n*p, where n is the number of people you know and p the probability of a particular person being an INTJ.
The probability of knowing exactly k INTJs out of n people is then n_Choose_k*p^k*(1-p)^(n-k).
If you want to know the probability of knowing 1 or more INTJs, then you simply add up prob(k out of n being INTJ) for all k from 1 to n. Or, more easily, subtracting prob(0 out of n being INTJ) from 1.
Let's say you know a 50 people and that the probability of a particular person you know being INTJ is 0.03, and that the probability of any one person you know, being INTJ is independent of any of the others being INTJs.
Then you would expect to know 1.5 INTJs on average. Right now I know 3 people who I suspect could be INTJs, one I am fairly certain is an ISTJ instead.
Also, the probability of knowing at least 1 INTJ is 1-prob(knowing 0 INTJs), which is then 1-(0.7)^50=approx. 1-1.8e-8, which almost nearly 1. So the real rarity would be to find someone who knew 50 people but didn't know an INTJ.
If you are in an NT skewed environment, like college or in an INTJ skewed environent like IT, then you are likely to know more.
When I think about peope you've known in the past, that number will go up. When I think about my high-school Math team, and the local chess club, the number could be high indeed.