Two goals only.
If Frankfurt loses, it will have only 19 goals difference at most, If Borussia wins by 2 it will also go up to 19 but they already have 3 more goals scored overall than them.
So, it looks perfectly doable....unless...
I would probably laugh for the entire next season if Dortmund loses at Kiel! 
Actually it's 3 goals. Because if Frankfurt loses by 1 goal but in a VERY high scoring game like 6:7 they would still be ahead of Dortmund. I know a result is unlikely but mathematically a 2 goal difference win can be not enough. So saying 3 goal difference and nothing else matters is the right wording. But yeah, I am also scared BVB will fumble it yet again on the 34th day, just like 2 years ago at home against Mainz.
Oh and by the way, BVB isn' playing at Kiel, they are facing them IN Dortmund.