Count of Related Tasks to Account

Try with below code.

trigger CountTasks on Account (before insert, before update) {
    // Put all Account into a Set
    Set<id> allAccIds = new Set<id>();
    for (Account acc : Trigger.new) {
        allAccIds.add(acc.ID);
        System.debug('ID added: ' + acc.ID);
    }
    //Query All Tasks to find Tasks with matching IDs
    List allTasks = [SELECT Id, Status, WhatID from Task
                           WHERE WhatID IN :allAccIds];
    System.debug('allTasks is ' + allTasks.size());
    List TaskArray = new List(); //All Tasks   
    Map elCountAll = new Map(); //Count of All Tasks
    for (Task u : allTasks) {
        TaskArray.add(u.WhatID);
    }  
    System.debug(allTasks.size());    
    // Get the matching tasks from the Map - and count Status
    //Start with our Account
    for (Account acc : Trigger.new) {
        //Count all Tasks
        for(String key : TaskArray)
        {
            if(!elCountAll.containsKey(key)){
                elCountAll.put(key,0);
            }
            Integer currentInt=elCountAll.get(key)+1;
            elCountAll.put(key,currentInt);
        }
        acc.countofAllTasks__c = elCountAll.get(acc.ID);
    }
}