Variable to change loop length each time it's called.

I have two random numbers that determine how many images are shown in two columns.

$set1 = mt_rand(1,$max);
$set2 = mt_rand(1,$max);

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I forgot how to make a modded variable to change the loop length each time it's run:

 for ($z = 1; $z <= 2; $z++){ 
for ($pieces = 1; $set.$z; $pieces++){     
      $device .= "<img src= {$direct}{$dfiles[$pieces]} alt=''/>
      ";
    }
}

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can somebody help?