To explicitly limit a linked-record field to one record, go to:
Form Fields
Click on a Linked Records field
Advanced section
Select Single Record in Allow users to link
Depending on the option selected here, the field will show either "single record" or "multiple records" in the fields list, so you can more easily tell which fields are set to which behavior.
Airtable Field settings
The miniExtensions setting takes precedence over Airtable’s single-record preference. Choose Single record here to enforce a one-record limit. If no miniExtensions override is stored, the form falls back to the Airtable preference.
Creating records
Single record limits this field to one linked record at a time. While a record is linked, users cannot create another record through this field.
Related
To see the setting imported from Airtable, open Advanced > Airtable Field Settings. You can view it there, but not change it. To set the miniExtensions limit, use Allow users to link above. See how Airtable field settings are imported for more detail.