How common is splenic rupture with sepsis?

Very rare. It happens, but it is rare enough that it usually gets reported in the medical literature when it does. Splenic rupture with mononucleosis is more common.
Very rare. In 35 years as an infectious disease specialist I have treated 1000's of cases of sepsis and have never seen a splenic rupture in any of them. I have seen splenic rupture in mononucleosis caused by epstein-barr virus.