For information on setting up Mailchimp for the first time, check out this guide.

When importing Contacts to Mailchimp, you need to set which Company Types and Contact Roles for the contacts you want to be synced. The number of contacts shown that fit the Company Type and Contact Role criteria also includes those who are 'Unsubscribed', which means their Email Flag is set to 0.

These unsubscribed Contacts won't be imported in the first import, but will be brought in with subsequent imports if they've been updated. For Contacts to be imported in the first run, their Email Flag should be set to 'Yes'.

After the initial import, subsequent syncs will run every 15 minutes. This will get a list of CRM Contacts that have been updated since the previous import, and then update/create these in Mailchimp. If an unsubscribed Contact has been updated, it'll be imported to Mailchimp, but will be marked as Unsubscribed.

The CRM will also filter out Contacts if they don't have an email address.