Why can I not add more than 1,000 leads or contacts to a MailChimp mailing list?
Adding or removing contacts from a mailing list via MailChimp integration is limited to 1,000 records at a time.
Depending on the number of contacts that you have, one workaround is to keep track of which Contacts have been exported to MailChimp. To do this, try selecting 1,000 and adding a bulk tag (see Adding and removing 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.
For more information, see the article: Using Mailchimp in Sell.