Hmmm....that is a bit trickier, as there is not an easy way to do that with sql (you could write a trigger, but that is an exercise left to the reader). What I might do in this situation is create a hash table and keep track of the number of times the first field has been seen. Some code (assume | delimited fields)

#!/bin/perl -w
use strict;

open(FILE,"myfile") or die "Couldn't open myfile";
my %first_record_hash;
while (<FILE>)
{
my @array = split '|';
if ($first_record_hash{$array[0]} < 6)
   {
   #insert record into database
   $first_record_hash{$array[0]}++;
   }
else
   {
   next;
   }
}
[download]

I think that'll do the trick for you