In support of atakacs...
Looking at official stats from Jan-Nov 2012, we see that passenger numbers grew by 7.8% at Tegel and by 0.3% at Schoenefeld, or 5.6% across the 2 airports (remember that Tegel is much larger than Schoenefeld)
Assuming Dec 2012 shows a similiar growth rate compared to Dec 2011, this would indicate Berlin saw about 25.4m passengers in 2012
Looking at growth rates over the last 5 years, one might hypothesise that passenger traffic would be between 26.5 and 27m passengers for 2013
Figures I've seen suggest that the new airport terminal has a design capacity for about 27m passengers, so the airport is likely to be kept busy from the day it opens.
They have enough runnway capacity, the question is over terminal capacity, remember it can be expanded to deal with more passengers (sadly they did not do it in the first place, they might as well...)