How to get the Incumbent value?

Summary:

We're looking a rule to prevent create job requisition when a specific position head count is 2 and we have 2 incumbents already active in that position, we don't want to create a new requisition for the same position. We would like to validate and throw error message.

Content (please ensure you mask any confidential information):

We have configured the field validation rule.

Version (include the version you are using, if applicable):

23D

Code Snippet (add any code snippets that support your topic, if applicable):