currently there is no way to count these without doing so manually.
In the 6.6 and 6.7 CP, however, you can make it easy
cp.cgi?page=membermanage;tab=userposts;u=00000001;pg=0;mode=1
will list all the topics the user has created that still exist
cp.cgi?page=membermanage;tab=userposts;u=00000001;pg=0;mode=0
will list all the posts the use has made.
Subtract those totals to see how many were replies