This is to address `b)` in the parent ticket legacy/trac#15714. It shouldn't be a fixed value else an attacker can count the number of intros required to rotate the intro point to gauge the popularity of a hidden service. See comment 1 and 2 from the parent ticket on this subject. https://trac.torproject.org/projects/tor/ticket/15714#comment:1 https://trac.torproject.org/projects/tor/ticket/15714#comment:2