Currently, the Content Relationship field does not allow you to link more than one document.
But it's possible to have many Content Relationship fields and select multiple documents using a different approach.
How to achieve this
This can be achieved using a Group field. In the Group field, you can put the Content Relationship field. This allows you to add as many Content Relationships as you want. So one Group field can have many Content Relationships.
How to get linked content from a Group field
FetchLinks does not support the Group field. If you need to retrieve fields from a Content Relationship inside of a Group field, it can be accessed by using the advanced version of fetchLinks called GraphQuery. Here is the developer documentation for GraphQuery: