list<Customer_Tool__c> ctListA = new List<Customer_Tool__c>();
list<Customer_Tool__c> ctListB = new List<Customer_Tool__c>();
Set<id> ctID = new Set<id>();

for(Attachment at : trigger.new) {
    ctID.add(at.parentid);
}

ctListA = [Select id, CTAttachment__c from Customer_Tool__c where id in: ctID];
    if(ctListA!=null && ctListA.size()>0){
        for(Customer_Tool__c ct : ctListA) {
            ct.CTAttachment__c = 0;
            ct.CTAttachment__c++;
            ctListB.add(ct);
        }
        update ctListB;
    }