instruction: count the number of times that patient 20713 had had a tooth extraction nec procedure.
response: SELECT COUNT(*) FROM procedures_icd WHERE procedures_icd.icd9_code = (SELECT d_icd_procedures.icd9_code FROM d_icd_procedures WHERE d_icd_procedures.short_title = 'tooth extraction nec') AND procedures_icd.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 20713)