Hello,
I am trying to include a Case Number of an email template for an email-to-case.
Basically, when a client sent an email to a closed case, he received an automatic email reply.
On the template I have added the merge field value {!Case.CaseNumber}
but the Case Number does not appear on the email reply sent to the client



I believe that, to add the Case Number in the automatic email reply, I need to reference the CaseId somewhere in my code. But I have figured out how.
The tried to do 

mail.setTargetObjectId(c.Id);

public class EmailManager {
    @future
    public static void sendEmailToCaseDeactivated(Set<Id> caseIds){
    
        List<Messaging.SingleEmailMessage> allmsg = new List<Messaging.SingleEmailMessage>();
        List<Case> lstCase = [SELECT Id, Status,ContactEmail FROM Case WHERE 
                              Status = 'Closed' AND
                              ContactEmail != Null AND
                              Id IN: caseIds];
        EmailTemplate templateId = [SELECT Id FROM EmailTemplate WHERE DeveloperName =:'Email_to_Case_Closed'];
       
        for(Case c : lstCase) {
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            mail.setTargetObjectId(UserInfo.getUserId());
            mail.setTemplateId(templateId.Id);
            String[] sendingTo = new String[]{c.ContactEmail}; 
            mail.setToAddresses(sendingTo); 
            mail.setSaveAsActivity(false);
            allmsg.add(mail);
        }               
        Messaging.sendEmail(allmsg, false);
    }
    
}

trigger IncomingEmailClosedCase on EmailMessage (before insert,after insert) {
    
    if(trigger.isBefore) { 
        Set<Id> sCaseIds = new Set<Id>();
        for(EmailMessage em: Trigger.New) {
            if (em.Incoming)
                 sCaseIds.add(em.parentId);   
        }
        if(!sCaseIds.isEmpty())
            EmailManager.sendEmailToCaseDeactivated(sCaseIds);
    }
    if(trigger.isAfter){
        Map<Id,Id> caseIdsToEmailMessageIds = new Map<Id,Id>();
        for(EmailMessage em: Trigger.New) {
            if (em.Incoming)
                caseIdsToEmailMessageIds.put(em.parentId,em.Id);   
        }
        set<Id> sMessageIdsToDelete = new Set<Id>();
        if(caseIdsToEmailMessageIds.isEmpty()) return;
        for(Case lstCase : [SELECT Id, Status,ContactEmail FROM Case WHERE 
                          Status = 'Closed' AND
                          ContactEmail != Null AND
                          Id IN: caseIdsToEmailMessageIds.keyset()]){
            if(caseIdsToEmailMessageIds.containsKey(lstCase.Id)){
                Id emId = caseIdsToEmailMessageIds.get(lstCase.Id);
                sMessageIdsToDelete.add(emId);
            } 
        } 
        if(!sMessageIdsToDelete.isEmpty())   delete [ select Id from EmailMessage where id in :sMessageIdsToDelete];
    }
}