How many patients with severe thrombosis have a normal prothrombin time?
severe thrombosis refers to Thrombosis = 2 or 1; normal prothrombin time refers to PT < 14;
SELECT COUNT(T1.ID) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID INNER JOIN Examination AS T3 ON T1.ID = T3.ID WHERE T2.PT < 14 AND T3.Thrombosis < 3 AND T3.Thrombosis > 0