First off I would clone the object using a number field and not a text field for Quantity. In the example below lets assume the SObject you were wanting to clone was named Service__c.

trigger AfterServiceUpdate on Service__c (after insert, after update) 
{
	//instantiate list of cloned services to insert
	Service__c[] services = new Service__c[]{};

	//iterate through updated sprockets
	for(Service__c service : Trigger.new)
	{
		Service__c oldService = (Trigger.isUpdate) ? Trigger.oldMap.get(service.Id) : null;

		//check to see if the quantity has changed
		//this logic could be expanded to find the difference of the two quantities
		//and create the appropriate amount of records based on the difference
		if(Trigger.isInsert || (Trigger.isUpdate && service.Quantity__c != oldService.Quantity__c))
		{
			//clone the record Quanitiy__c number of times
			for(Integer i=0; i<service.Quantity__c; i++)
			{
				Service__c clonedService = service.clone();
				clonedService.Quantity__c = 0;
				services.add(clonedService);
			}
		}
	}

	//add cloned services
	if(services.size() > 0) insert services;
}

And here is a basic unit test to verify to works:

@isTest
private class AfterServiceUpdateTest {
	
	@isTest static void test_method_one() 
	{
		Service__c s = new Service__c();
		s.Quantity__c = 0;
		insert s;

		Test.startTest();

		s.Quantity__c = 2;
		update s;

		Service__c[] allServices = [SELECT Id FROM Service__c];
		//there should now be a total of 3 services since we updated the quanity to 2
		system.assertEquals(3, allServices.size());

		Test.stopTest();
	}
	
}

 Let me know if you have any questions.