records into internal table

hi,

i have a field in selection screen called p_count

and i and getting records from marc table and downloading it to presentation server.p_count is having a default value 100(p_count is used to select no of records)

if i give p_count as 100 will get 100 records from marc after all condition check. but usually there will be only 40 records which satisfy all the condition in select query

so even if i give 100 will get only 40 records .now the problem is if i give p_count as 20 i want only 20 records even if 40 records are there.how can we delete excess record ie out of 40 records which satisfy the condition i only need 20 which i am giving in p_count.in some condition p_count can vary to any number.

please give a solution.

thanks,

gopan