Here’s a Migrate API process plugin I wrote in Drupal 9 that skips a property or row when more than one value exists. My use case:

1. My source data has locations; each location has multiple associated organizations
2. My new Drupal site has locations; each location has a parent organization
3. I want to only populate the field_parent_org field (an entity reference) if there is a single organization value in the source data for the location
4. I’m using a custom module called “pdms_migrate”

I’ve stripped out all of the noise from the examples below… hopefully it’s enough to help you understand the example: