I cannot add more than 1,000 Leads or Contacts to a Mailchimp mailing list?
At this time, Contacts added to or removed from a mailing list is limited to 1,000 Contacts at a time for the MailChimp integration.
Depending on the number of Contacts that you have, a workaround would be to keep track of which Contacts have been exported to MailChimp. To do this, try selecting 1,000 and adding a bulk tag (How do I add or remove Tags on Leads, Contacts and Deals?). Once the tag is added, you can send those tagged Contacts to your Mailchimp mailing list.
Then, you can filter your smart list to include all contacts WITHOUT the tag. Once it's filtered, perform the same action on the next 1,000. Repeat until you've exported all of your desired Contacts into MailChimp.
To learn more about the integration, visit our article on the Mailchimp Integration.