// in a for / foreach loop
$count = 0;
foreach($something as $something_else)
{
  //exec stuff here
  $count++;
  if($count % 10 == 0) echo "<br />";
}

// in a while loop
$count = 0;
while(condition)
{
  //exec stuff here
  $count++;
  if($count % 10 == 0) echo "<br />";
}