What are th chances of a recurring pe or clot after 6 months on warfarin and other wise healthy 21 y/o?

It depends: It depends on what caused the first episode. If it's due to a knee surgery or a pregnancy, the chances are not very high. The picture changes if you have a genetic anomaly that makes you more prone to have clotting problems.

10+%: Depending on which study you look at, the risk is at least 10% even if the pe was provoked by a transient factor (immobility, pregnancy etc). Data shows that indefinite anticoagulation significantly decreases the risk of subsequent pe but at an increased risk of bleeding.