How to add multiple footer widgets?

Footer 1 and Footer 2 are sidebars you can add your own sidebar in functions.php or create a child theme :

function twentyseventeen_widgets_init() {
register_sidebar( array(
    'name'          => __( 'Blog Sidebar', 'twentyseventeen' ),
    'id'            => 'sidebar-1',
    'description'   => __( 'Add widgets here to appear in your sidebar on blog posts and archive pages.', 'twentyseventeen' ),
    'before_widget' => '<section id="%1$s" class="widget %2$s">',
    'after_widget'  => '</section>',
    'before_title'  => '<h2 class="widget-title">',
    'after_title'   => '</h2>',
) );

register_sidebar( array(
    'name'          => __( 'Footer 1', 'twentyseventeen' ),
    'id'            => 'sidebar-2',
    'description'   => __( 'Add widgets here to appear in your footer.', 'twentyseventeen' ),
    'before_widget' => '<section id="%1$s" class="widget %2$s">',
    'after_widget'  => '</section>',
    'before_title'  => '<h2 class="widget-title">',
    'after_title'   => '</h2>',
) );

register_sidebar( array(
    'name'          => __( 'Footer 2', 'twentyseventeen' ),
    'id'            => 'sidebar-3',
    'description'   => __( 'Add widgets here to appear in your footer.', 'twentyseventeen' ),
    'before_widget' => '<section id="%1$s" class="widget %2$s">',
    'after_widget'  => '</section>',
    'before_title'  => '<h2 class="widget-title">',
    'after_title'   => '</h2>',
) );
}
add_action( 'widgets_init', 'twentyseventeen_widgets_init' );

you can add as many widgets as you want by drag and drop widgets. The template that displays in twentyseventeen/template-parts/footer/footer-widgets.php

<?php
 if ( is_active_sidebar( 'sidebar-2' ) ||
 is_active_sidebar( 'sidebar-3' ) ) :
?>

<aside class="widget-area" role="complementary" aria-label="<?php esc_attr_e( 'Footer', 'twentyseventeen' ); ?>">
    <?php
    if ( is_active_sidebar( 'sidebar-2' ) ) { ?>
        <div class="widget-column footer-widget-1">
            <?php dynamic_sidebar( 'sidebar-2' ); ?>
        </div>
    <?php }
    if ( is_active_sidebar( 'sidebar-3' ) ) { ?>
        <div class="widget-column footer-widget-2">
            <?php dynamic_sidebar( 'sidebar-3' ); ?>
        </div>
    <?php } ?>
</aside><!-- .widget-area -->

<?php endif; ?>