Go to US news, get the number of interviewees, and the number of acceptances.

Let P_i be the ratio of acceptance to number of interviewees for each school.

The probability that you will get accepted somewhere is equal to

A = 1 - (1-P_1) x (1-P_2) x (1-P_3) x ...(1-P_i)

So if you had 3 IIs, where the fraction accepted of interviewees are 0.25, 0.3, and 0.5, the probability that you will get accepted SOMEWHERE is 1-(1-.25)(1-.3)(1-.5) = 0.7375 ~ 73% chance.

Well if you want a specific number, I would say 4. My reasoning is that since most schools have about 25% acceptance after an interview, you have a decent chance of getting into at least 1 out of the 4 (given you have basic social skills).

Four schools at 25% each actually gives a 57% chance of getting in at least one place. So, yeah, pretty good chance, but still just slightly better than a coin flip.

EDIT: nevermind, it's 68%