How to Set a Maximum Upload Images in WordPress?

Good idea.

Try adding this code in your theme functions.php or even better create a new plugin and paste the code there

define("LIMIT_FILE_UPLOADS", 2);

add_filter("adverts_gallery_upload_prefilter", "limit_file_uploads");

function limit_file_uploads( $file ) {

    if ( !isset($file["name"]) || !isset($file["type"]) ) {
        return $file;
    }

    if ( !isset( $_POST["post_id"] ) ) {
        $post_id = 0;
    } else {
        $post_id = intval($_POST["post_id"]);
    }

    if( $post_id < 1 ) {
        // first image upload.
        return $file;
    }

    $attachments = get_children( array( 'post_parent' => $post_id ) );
    $images = count( $attachments );

    if( $images >= LIMIT_FILE_UPLOADS ) {
        $file["error"] = sprintf( "You cannot upload more than %d images.", LIMIT_FILE_UPLOADS );
    }

    return $file;
}

In the first line change define("LIMIT_FILE_UPLOADS", 2); change 2 to max number of images (per ad) you want users to be able to upload.

Thank you.