if the source is flat file we don't generate any SQL query in the source qualifier. then we are using the following way, seqgen | sd-->sq-->expr-->filter--> td. in the filter condition expression we take nextval<=52000.
you have 100000 records and you want to load 52k records in the target. then, source definition--> source qualifier-->target definition. open source qualifier properties select SQL query,in that field generate the query with WHERE clause rownum<=52000. and run the session.