Write a trigger to prevent an Employee__c record from being inserted if the parent Department__c already has 10 active employees.

trigger EmployeeTrigger on Employee__c (before insert) {
    Set<Id> deptIds = new Set<Id>();
    for (Employee__c e : Trigger.new) if (e.Department__c != null) deptIds.add(e.Department__c);
    Map<Id, Integer> counts = new Map<Id, Integer>();
    for (AggregateResult ar : [SELECT Department__c d, COUNT(Id) c FROM Employee__c
                               WHERE Department__c IN :deptIds AND Active__c = true GROUP BY Department__c]) {
        counts.put((Id) ar.get('d'), (Integer) ar.get('c'));
    }
    for (Employee__c e : Trigger.new) {
        if (e.Department__c == null || !e.Active__c) continue;
        Integer n = counts.containsKey(e.Department__c) ? counts.get(e.Department__c) : 0;
        if (n >= 10) e.addError('Department already has 10 active employees.');
        else counts.put(e.Department__c, n + 1);
    }
}