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 through the 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, select 1,000 and add a bulk tag. Once the tag is added, send those tagged contacts to your MailChimp mailing list.
Then, 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.