If you have over thousands and thousands of records in your _wc_session_ in your wp_options table, this could be a sign cron job is not running properly and search engine bots are putting stuff in their WooCommerce shopping bags. Here is what to do.
1. Clear these records using WooCommerce
Open your WordPress dashboard then go to:
Woocommerce > System Status > Tools, "Clear all sessions"
to clear the sessions.
2. Fix cron, and make sure cron job is running properly. Best to consult with your hosting company on this one. Clue: mod_sec (firewall) could be terminating the cron job.
3. Make sure search engine bots do not stuff in their shopping carts. In your robots.txt file add the following lines:
User-agent: * Disallow: /*add-to-cart=*
To read more about solutions visit https://wordpress.org/support/topic/a-lot-of-wc-sessions
Photo credit to Justin Morgan at Flickr.
If the problem persists, think about opening a support ticket with WooCommerce.