wc_orders_count() from includes/wc-order-functions.php file.

wc_orders_count.php

<?php

/**
 * Return the orders count of a specific order status.
 *
 * @param string $status
 * @return int
 */
function wc_orders_count( $status ) {
        $count          = 0;
        $status         = 'wc-' . $status;
        $order_statuses = array_keys( wc_get_order_statuses() );

        if ( ! in_array( $status, $order_statuses ) ) {
                return 0;
        }

        $cache_key = WC_Cache_Helper::get_cache_prefix( 'orders' ) . $status;
        $cached_count = wp_cache_get( $cache_key, 'counts' );

        if ( false !== $cached_count ) {
                return $cached_count;
        }

        foreach ( wc_get_order_types( 'order-count' ) as $type ) {
                $data_store = WC_Data_Store::load( 'shop_order' === $type ? 'order' : $type );
                if ( $data_store ) {
                        $count += $data_store->get_order_count( $status );
                }
        }

        wp_cache_set( $cache_key, $count, 'counts' );

        return $count;
}