For MailChimp API 2.0 there was a method \'batch-subscribe\', to send in an array of email addresses to be added to a specific list in MailChimp. How to implement this i
This is not an issue at the Mailchimp end. You just need to use arrays and objects properly.
Good batch subscribe example you can find here https://rudrastyh.com/wordpress/wp-users-to-mailchimp-list.html#batch_subscribe_php