2 : 18 positions
4 : 11
6 : 3
The first column shows the number of times a position was used in a game. In other words, 18 SPs were used twice, and so on. Since each SP was played twice, each engine in turn taking White, this is the minimum. I once attempted to calculate how much each of the 960 SPs were scrambled relative to the traditional start position. The results were summarized in A Chess960 Almanac (October 2011).
Is there a correlation between 'unbalanced books' and the degree of scrambling? A preliminary analysis was inconclusive, but 32 different SPs is too small a sample to decide one way or the other. I hope we'll eventually get enough data to say for sure.