how many times during their first hospital visit is patient 24762 getting a ptt test?
select count(*) from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ptt' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 24762 and admissions.dischtime is not null order by admissions.admittime asc limit 1 )