count the number of times that patient 20713 had had a tooth extraction nec procedure.
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 )