If you're willing to be a sister with just about any house, the chances are probably very good. It has happened that people will sometimes get cross cut, or get dropped because they don't stand out or do something horrendous by accident, but that's pretty unlikely.

If you're committed to join a sisterhood, there's a good chance a sisterhood will have you.
Sometimes people will say "I'll be willing to look at these ten of the twelve organizations" because the last two are really tiny and are rebuilding or going through a crisis or something. If you get kept by all of your dream houses, then you may be able to pick and choose, but you may want to return to any house that invites you back (if you get fewer or equal to the maximum houses for each successive round), just to make sure they get a better chance to know you and that you get a better chance to get to know them.
If things don't work out through formal, your school may have orgs that have open slots and will COB. Again, these are often the sisterhoods with small numbers due to a huge senior class the previous year, or a general rebuilding/restructuring going on. Lots of times people encourage others not to look at these orgs, but you need to make your own decision. It could be a disaster for you, or it could be your home, and your joining the org could provide them with the shot they need to create more interest.
I'm not saying you have to settle for a "lesser organization," but the orgs with problems are usually dying to take members. The pickier you get, or the more/earlier you set your sights to a certain narrow group of sororities, the more you will limit your options.
Hope that makes sense.