We have not had default count null or non-null for column yet so here is a solution:

<?php
$count_mycolumn = 0;
$this->dataStore("mystore")->popStart();
while($row = $this->dataStore("mystore")->pop())
{
    if($row["mycolumn"]!=null)
    {
        $count_mycolumn++;
    }
}
Table::create(array(
    "dataSource"=>$this->dataStore("mystore"),
    "showFooter"=>"bottom",
    "columns"=>array(
        "mycolumn"=>array(
            "footerText"=>"$count_mycolumn";
        )
    ),
    ...
));
?>

As you can see, the above code will read dataStore and count the non-null in mycolumn. Then it show at footerText of that column. You can use number_format() to format the count number as you wish.

Hope that helps.

Please let us know if you need further assistance.