This option has to be set manually in the linked records field settings on miniExtensions to limit it to only accepting a single record. You can find it by going to:
Linked field settings
Record Finder Options
Allow selecting multiple records
If your field settings on Airtable have the "Allow linking to multiple records" option disabled, users will still be able to add multiple linked records on the form even if the field on Airtable is set to only allow a single linked record.
When this option is disabled, users cannot select (or create) multiple records. Although this setting might indicate that it only affects "selecting". However, it also affects "creating" multiple records (if enabled).
On the legacy site app.miniExtensions.com
Form (Edit & Add Records in the portal)
Click on the linked record field
Allow users to select multiple records