guarantee your spot in a NY6 bowl game.
I guess technically if all 7 "auto-bids" ended up outside the top 12, then the 5 "at-large" bids would be teams ranked #1-#5. That's never going to happen, but HYPOTHETICALLY it could happen.
This season, it's highly plausible that the Big 12 champion, PAC-12 champion and/or the ACC #3 team (assuming both Clemson and Notre Dame make the playoff) end up outside of the top 12. If they all end up ranked that low, then it would take finishing in the top 9 to get into an NY6 bowl as an at-large (and would mean 8-10 SEC/Big Ten teams in the top 12).
Basically I would say that a top 8 finish guarantees an NY6 bid, and it would be highly unlikely that a top 10 team would be left out. Finish #11 or #12 and things are a bit dicey for NY6 inclusion, though.