It is very hard for anyone to say what number is 'good or bad', or what your chances are because it all depends on how many students guaranteed res change their mind.
If your number is >100 I would start planning ahead and looking at off-campus houses, there are a LOT out there and they are great! Plus there are many first-year students who are looking for roommates to go into a student house.
Check the MacInsiders Housing Marketplace here:
http://www.macinsiders.com/forumdisp...196.html?f=196 and look for students that posted looking for first-years to move in together with them
There's also the Facebook mcmaster off-campus housing postboard here:
https://www.facebook.com/groups/machousing/
Also check the off-campus housing resource centre
http://macoffcampus.mcmaster .ca