To maintain variable value inside the Batch class, Database.Stateful is used.


Sample Class:

global class Class_Name implements Database.Batchable<sobject>, Database.Stateful{
    Integer i = 0;
    global Database.QueryLocator start(Database.BatchableContext bc){
        return Database.getQueryLocatory(‘SELECT Id, Name, Sequence_Number__c FROM Employee__c’);
    }
    
    global void execute(Database.BatchableContext bc, List<Employee__c> listEmployee){
        for(Employee__c e : listEmp){
            e.Sequence_Number__c = i;
            i += 1;
        }
    }
    
    global void finish(Database.BatchableContext bc){
    }
}


here i value will be maintained even though execute method is called several times.

Cheers!!!