In Prismic, you can create a one-to-many relationship by putting a Content Relationship field inside of a Group field. This way, your editors can add multiple Content Relationships within one field.
How to achieve this
First, create a Group field. In the Group field, add a 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: