How to create person number validation while hiring an employee?

While hiring an employee in the system using 'Hire an employee' quick action, and when the person number generation method is set to manual, when users are entering the person number it should have a validation - the number that they enter should be the last employee number in the system+1 based on legal employer person number series. If the validation is successful then it should allow to hire the employee.

Example:

If we have 3 legal employers A, B. Person number series for legal employer A starts from 1000. Person number series for legal employer B starts from 2000. Then if the employee is getting hired in legal employer A, it should validate the number entered by the users if it is equal to last employee number in legal employer A+1.