This would only be possible for editing a record. You cannot have an attachment unless the parent already exists.

This class will return the number of attachments and you can process as you need

public class checkAttachments{

  public static boolean isInsert = false;
      public static Map<ID,integer> numberOf(SObject[] obj){
           AggregateResult[] att = [Select count(ID)cnt, ParentID From Attachment Where ParentID IN :obj GROUP BY ParentID ];
          
          Map<ID,Integer> mOfAttachmentCount = New Map<ID,Integer>();

          for(AggregateResult a : att){

              mOfAttachmentCount.put((String)a.get('ParentID'),(Integer)a.get('cnt'));

          }

          
          for(SOBject o : obj){

              if(!mOfAttachmentCount.containsKey(o.id))
                  mOfAttachmentCount.put(o.id,0);

          }

          return mOfAttachmentCount;

      }

}

From your trigger pass in the trigger.new object from a before update trigger event.

**On the insert event call checkAttachments.isInsert = true. Then check for the checkAttachments.isInsert == false in your trigger to add error. This will prevent the insert from failing when workflows fire off the update event (if you have them)