what is a number of times that patient 17638 has had an l nephrostomy's output in 05/2105?
select count(*) from outputevents where outputevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 17638 ) ) and outputevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'l nephrostomy' and d_items.linksto = 'outputevents' ) and strftime('%y-%m',outputevents.charttime) = '2105-05'