When you read the documentation it confirms that a communication record will be created as part of the synchronization of data from Mailchimp to Sage CRM.
"A communication record is created for each email recipient. Your system administrator specifies how often communication records are created. Click the Communications tab for the relevant company, person, or lead record to view information about the Mailchimp campaign email that they received. "
This is just a very brief statement and it is worth expanding on this a little more.
There actually two synchronizations between Sage CRM and Mailchimp. (Partial and Full).
The partial synchronization will only give the summary data (Who clicked, Opened etc).
The full synchronization will give you the communications and then synchronize the opt outs. Within Sage CRM this is set to 24 hours. Therefore after 24 hours it should synchronize the communications and opt outs from Mailchimp down to CRM.
The context in which the campaign was created (Company/Lead/Person) will determine where the communication will be created.
If a Campaign was created using person records then the communication will be added against the person record and not the company record. The same is for the lead record.
You need to check that you are looking at communication in the correct context, but everyone that was uploaded to that campaign (and therefore the Mailchimp list segment) should get a communication record, provided that a full synchronization took place.