I'm going to assume they go pretty high. My cryptarch rank is 24

i have rank 10 and it`s gonna be rank 11 as soon as the nightfall and heroic resets from what i`ve seen so far it gives you every 3 ranks a weapon plus the usual stuff (shards, energy) and whats also special is, that i`ve got a blue sparrow after hitting rank 8 hope this helps to satisfy your curiosity a little

I know someone who claims to be at rank 9.