How to get the Page Number of the Current Page in Single Post Pagination

For your single page, you could try using the global $page variable.

Another alternative would be to try fetching the query page variable, with e.g.:

$wpse_current_page = (int) get_query_var( 'page' );

if( $wpse_current_page > 0 ) {
    printf( 
        __( 'Page Number: %d', 'wpse-theme-slug' ), 
        (int) $wpse_current_page  
    );
}

on your single page.