Is this what you mean:

trigger GeneralCaseAlertNotification on Case (after insert, after update) {
if(RunTriggerOnceEmail.firstRun){

    //Boolean used to send email only if valid conditions and email exists
    Boolean toMail = false;
    List<string> lstEmail = new List<string>();
    Set<id> setCaseId= new Set<id>();
    Set<id> setAccountId = new Set<id>();
    Set<String> setEmail = new Set<String>();
    String salesforceUrl = URL.getSalesforceBaseUrl().toExternalForm() + '/';

    Map<Id, User> mapUnSubscribedUser = new Map<Id, User>([Select 
                                                            Email_Alert_Subscription__c , 
                                                            Id 
                                                            From 
                                                            User 
                                                            where 
                                                            Email_Alert_Subscription__c 
                                                            includes ('General CIQ Case Alert') 
                                                            And
                                                            isActive = true]);

    for(Case objcase: trigger.new){
    if(Trigger.IsInsert && objcase.Status == 'Escalated'){
        setCaseId.add(objcase.id);
        setAccountId.add(objcase.AccountId);
    }
    
    if(Trigger.IsUpdate && trigger.oldMap.get(objcase.Id).Status != objcase.Status && 
        (objcase.Status == 'Closed' || objcase.Status == 'Escalated'))
        { 
            setCaseId.add(objcase.id);
            setAccountId.add(objcase.AccountId);
        }
    }
    
    Map<Id, List<AccountTeamMember>> mapATM = new Map<Id, List<AccountTeamMember>>();
    for(Account a : [Select Id, 
                    (select user.email from AccountTeamMembers where 
                    TeamMemberRole 
                    IN('Account Manager',
                    'Lead Sales Executive',
                    'Regional Sales Executive',
                    'Team (Activity Alerts)')) 
                    from Account where Id In: setAccountId])

        mapATM.put(a.Id, a.AccountTeamMembers);

    List<Messaging.MassEmailMessage> lstMail = new List<Messaging.MassEmailMessage>();

    String strEmailBody = '';
    String strCaseDetail = '';

    for(Case objcased : [SELECT Id, CaseNumber, Type, Status, Origin, 
                        Subject, Priority, Description, Resolution__c, 
                        Action_Code__c, Resolution_Code__c, 
                        ID_Sharing_User_Names__c, Internal_Notes__c, 
                        ID_Sharing__c, Feature__c, Section__c,
                        Contact.Id, Contact.Name, 
                        Contact.Phone, Contact.Email,ThreadID__c, 
                        Account.Id, Account.Name, Ownerid,CreatedBy.Alias,
                        Owner.Name FROM Case 
                        Where Id IN: setCaseId
                        AND RecordType.Name = 'Client Services - CIQ'
                        AND CIQ_Escalate_To__c != 'Account Team']){
                        
                        
        lstEmail = new list<string>();
        setEmail = new Set<String>();
        strCaseDetail = '';

    //Collect Email Id from Account Team Member
    if(mapATM.ContainsKey(objcased.Accountid) && !mapATM.get(objcased.Accountid).isEmpty())
    for(AccountTeamMember objAtm : mapATM.get(objcased.Accountid))
    if(mapUnSubscribedUser.containsKey(objAtm.UserId))  
    
    setEmail.add(objAtm.user.email);

if(!setEmail.isEmpty()){
if(!toMail)
toMail = true;

//Get Unique Email ids
for(String s: setEmail)
lstEmail.add(s);

strEmailBody = 
'This is a general case alert. However, you can reply to this e-mail and the Case Owner will be alerted.'+'<br/>'+ 
'Please see below.'+'<br/>'+
'Case Owner:' +  objcased.Owner.Name + '<br/>'+
'Case Detail:'+'<br/>'+
'Type = ' + objcased.Type + '<br/>'+
'Status = ' + objcased.Status + '<br/>'+
'Origin = ' + objcased.Origin +  '<br/>'+
'Priority = ' + objcased.Priority +'<br/>'+
'Client Info:'+'<br/>'+
'Account Name = ' + 
'<a href=\'' + salesforceUrl + objcased.Accountid + '\'>'+
objcased.Account.Name +
'<a/>'+ '<br/>'+
'Contact Name = ' +
'<a href=\'' + salesforceUrl + objcased.Contactid + '\'>'+ 
objcased.Contact.Name + 
'<a/>'+ '<br/>'+
'E-mail = ' + objcased.Contact.Email +'<br/>'+
'Phone = '+ objcased.Contact.Phone +'<br/>'+
'General Product Information:'+'<br/>'+
'Section = ' +  objcased.Section__c +'<br/>'+
'Feature = ' + objcased.Feature__c +'<br/>'+
'Sharing? = ' + objcased.ID_Sharing__c +'<br/>'+
'Sharing with = ' + objcased.ID_Sharing_User_Names__c+'<br/>'+
'Description Information:'+'<br/>'+ 
'Subject =' +  objcased.Subject +'<br/>'+
'Description = ' +objcased.Description +'<br/>'+
'Resolution Details: Action=' + objcased.Action_Code__c+'<br/>'+
'Resolution =' + objcased.Resolution__c+'<br/>'+
'Internal Notes ='+ objcased.Internal_Notes__c +'<br/>'+
'Case number = ' + objcased.CaseNumber +'<br/>'+
'Thread Id : ' + objcased.ThreadID__c +'<br/>'+
'Click on the link to access the case: '+
'<a href=\'' + salesforceUrl + objcased.Id + '\'>'+
objcased.CaseNumber +
'<a/>'+ '<br/>'+
strCaseDetail;



              
OrgWideEmailAddress owa = [select id, DisplayName, Address from OrgWideEmailAddress 
                            where displayName='S&P Capital IQ Client Services' limit 1];




//New instance of a single email message
Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();

       // mail.settoAddresses(lstEmail);
        mail.setTargetObjectIds(lstEmail);
        mail.setSenderDisplayName(owa.id);
        //mail.setOrgWideEmailAddressId(owa.id);
        mail.setSaveAsActivity(true);
        mail.setSubject('General Case Alert, Case Number:' + objcased.CaseNumber + ', ' + objcased.Account.Name+', '+ objcased.Contact.Name);
        //mail.setHtmlBody(strEmailBody);
        mail.setTemplateID('00XQ0000000DzOb');
        if(objcased.CreatedBy.Alias!='Merge' || objcased.CreatedBy.Alias<>'Merge' ){
            lstMail.add(mail);
        }
}
}
    try{
        //Send Email
        if(toMail){
            
           Messaging.sendEmail(lstMail);
        
        }
    }
    catch(exception objEx){
        Trigger.new[0].addError('Some unexpected error occured. Please call your system admin if error persists : '+objEx.getMessage());
    }
    RunTriggerOnceEmail.firstRun=false;

}
RunTriggerOnceEmail.firstRun=false;
}

 How would I test this to see what limits am i hitting? What is the limit for mass emailing?