offset and max_num_pages in pagination gallery

I rewrote the code galleries and stopped at the pagination. How can I get the maximum number of pages? at the moment I can not define it, and put a test for the number 5.

 (Here "for ($ i = 1; $ i = 5; $ i ++)" here "if ($ paged 5))."

And how to make that work offset parameter in pagination? Now when I put the parameter offset, pagination will not work correctly. I need to post 16 pictures post_per_page was equal to 4. I have not found a solution for my specific case. Maybe it can be replaced with something? Here is my complete code in funсtions.php

    remove_shortcode("gallery");
add_shortcode("gallery","my_gallery");

function my_gallery($atts){
    $img_id = explode(',', $atts['ids']);
    if(!$img_id[0]) {
        return 'div class="no-img-gallery"No images/div';
    }

    $paged = get_query_var('paged') ? get_query_var('paged') : 1;
    $pictures = get_posts( array(
        'posts_per_page' = 4,
        'post__in'       = $img_id,
        'paged' = $paged,
        'post_type'      = 'attachment',
    ) );

    $html = 'div class="gallery-my"';
    foreach ($pictures as $item){

        $img_desc = $item-post_content;
        $img_caption = $item-post_excerpt;
        $img_title = $item-post_title;
        $img_thumb = wp_get_attachment_image_src($item-ID);
        $img_full = wp_get_attachment_image_src($item-ID,medium);


        $html .="
        div class='item-gallery'
            a href='{$img_full[0]}'
                img src='{$img_thumb[0]}' width='{$img_thumb[1]}' height='{$img_thumb[2]}' alt='{$img_title}'
            div class='desc-wrap'
                    strong class='title-img'{$img_title}/strong
                    span class='desc-img'{$img_desc}/span
                /div
            /a 
        /div";

    }

    $html .= "/div";
//Pagination
    $html .= "div class='pagination-gallery'";
        if ($paged  1) { 
            $html .="span class='prev-cust'
                        a href='?paged=".($paged-1)."'/a
                    /span/div";
        }
        for($i=1;$i=5;$i++){
            $html .="span class='num-pag'
                a href='?paged=".$i."'";

            if($paged==$i){
                 $usl='class="selected"';
            }
            else{
                 $usl='';
            }

            $html .=$usl."".$i."/a/span";
        }
        if ($paged  5) { 
            $html .="span class='prev-cust'
                        a href='?paged=".($paged+1)."'/a
                    /span/div";
        }

    $html .= "/div";                              


    return $html;

}