Ho risolto
codice:
$count = 0;foreach ($pdo->query("SELECT * FROM tabella") as $row) {

    <input type='checkbox' name='seleziona[{$count}]' value='{$row['id']}'/><br /></td><td>
    <input type='number' min='1' max='99' autocomplete='off' name='quantita[{$count}]' id='{$row['id']}' value='".substr($row['quantita'],3)."'/>
            $count++;
}
inserisci.php
codice:
foreach($_POST['seleziona'] as $k=>$v){
        $query = $pdo->prepare("INSERT INTO tabella1 (nome, cognome, prodotto, quantita) VALUES ('$nome','$cognome', (SELECT prodotto FROM tabella WHERE ID=$v), '{$_POST['quantita'][$k]}')");
        $query->execute();
    }