you don't need sed for that, try
#!/usr/bin/bash
line=`head -1 assembled fie_new`
c=1
while test $c -le 10
do
echo $line >> an_other_file
c=`expr $c + 1`
done
or
#!/usr/bin/bash
line=`head -1 assembled fie_new`
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file
echo $line >> an_other_file