Hi Ashu,

Please use the below code to get past days tasks related to Lead.

public class getPastTimeTaskRelLead {
    public static void getAllPastTimeLead(){
        Lead[] leadList = [SELECT id, Name from Lead LIMIT 50000];
        Task[] leadTasks = [SELECT id, subject, Whoid, LastModifiedDate FROM Task WHERE Whoid in: leadList AND LastModifiedDate = TODAY LIMIT 50000];
        System.debug('leadTasks:' + leadTasks);
        Map<Id, List<Task>> leadIdVsTaskMap = new Map<Id, List<Task>>();
        if(leadTasks.size()>0){
            for(Task taskObj : leadTasks){
                if(!leadIdVsTaskMap.containsKey(taskObj.Whoid)){
                    leadIdVsTaskMap.put(taskObj.Whoid, new List<Task>());   
                }
                leadIdVsTaskMap.get(taskObj.Whoid).add(taskObj);
            }
        }
        for(Lead leadObj : leadList){
            if(leadIdVsTaskMap.containsKey(leadObj.Id)){
                List<Task> getTaskList = leadIdVsTaskMap.get(leadObj.Id);
                System.debug('leadObj Name::::'+leadObj.Name+'----related number of task in past days::::'+getTaskList.size());
            }
        }
        
    }
}

I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks and Regards,
Sachin Arora
www.sachinsf.com