Why the value of the selector doesnt remain visible in the custom field after I edit the post?

There’s nothing in your code that would allow this to happen. In your HTML you need to add a selected attribute to the <option> element representing the current value. WordPress includes a helper function, selected() which can help with this:

function camere_meta_box_render( $post ){
    $number = get_post_meta( $post->ID, "function_camere", true );
    echo '<div><select name="function_camere">';
    for( $i = 0; $i < 13; $i++ ) {
    echo '<option value="'.$i.'" ' . selected( $i, $number, false ) . '>'.$i.'</option>';
    }
    echo '</select></div>';
}