apologies. i guess it was in the paper not the preso, uppr right of page 3 of C. Pelsser, L. Cittadini, S. Vissicchio, and R. Bush, From Paris to Tokyo: On the Suitability of Ping to Measure Latency, 2013 Internet Measurement Conference. <http://conferences.sigcomm.org/imc/2013/papers/imc125s-pelsserA.pdf> the intuition is that it is a function of the richness of the path diversity. perhaps a tunable? The way it looks to me is that that section argues that you need more than 6 and that 32 is enough. It doesn't really say that 16 is not enough :-)
it's been a while, and something happened to my memory but i forget what. but i suspect it is dependent on the diversity of the particular path.
But I just created an internal ticket to have the limit raised to 64. That should be ample for anybody who wants to experiment.
cool! randy