Avarage height of Red-Black tree
As a rule of thumb, your red-black tree depth can't be less than log(n) and it shouldn't be more than 2*log(n+1). Your numbers look correct. (log(1000)=10 and your 1000 sample tests are between 10 and 20).

Ref: http://lcm.csa.iisc.ernet.in/dsa/node115.html