WordPress Post Grid

This isn’t really a WordPress related question, and you can get your desired result by a simple PHP loop:

$i = 1;
foreach ($array as $var) {
    if ($i==7) {
        // six items already displayed. 

        // Do whatever you want here 

        // now restart the count
        $i = 1;
    }

    // display your thumbs or whatever

    // increment $i
    $i++
}