Yo probaría 2 cosas:
1)
Código PHP:
$qry=mysql_query(" SELECT COUNT(*) as cant FROM $tbl_name WHERE subrubro='Compra|Venta' ");
$row=mysql_fetch_assoc($qry);
echo $row['cant'];
2)Lo mismo pero sin el where:
Código PHP:
$qry=mysql_query(" SELECT COUNT(*) as cant FROM $tbl_name ");
$row=mysql_fetch_assoc($qry);
echo $row['cant'];