How many patients diagnosed with SLE have a normal white blood cell level?
diagnosed with SLE refers to Diagnosis = 'SLE'; normal white blood cell level refers to WBC between 3.5 and 9.0;
SELECT COUNT(T1.ID) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T1.Diagnosis = 'SLE' AND T2.WBC BETWEEN 3.5 AND 9