I don't have an official number, but the MSAR advisor report says that they typically accept 20 people off the waitlist of 50 people. (Not everyone is automatically placed on the WL post-II.) That gives a post-WL acceptance rate of 40%. However, there were at least 6 OOS students who were accepted off the WL last year (at least they claimed it on SDN), so there were less than 14 IS accepted students from the WL. That would mean 14/50 or roughly 28% for post-WL, IS movement. If we assume that not everyone uses SDN, then it is likely that there are more than 6 OOS students accepted off the WL. If that number was 50%-100% higher, the IS movement could be as low as 11 or 8 seats only, which would give you a 16%-22% chance of getting off the WL once you're put on as an IS applicant.
These are obviously just estimated though and it will vary each year.