TGJB,
I took a quick look at the last 5 Derbys and the 6 bearing in or out is about in line with most years.
Chrome and Bode each had a bo or bi line. Shackleford ran very well and he had 2 heading into the Derby.
The numbers, and I went up a dead end, were:
2014 7
2013 8
2012 4
2011 5
2010 7
Those numbers of starters either bore in or out during their 3 y.o. campaign.