public class QuoteLineItemHandler
{
	public void onBeforeInsert(List<QuoteLineItem> lstNewQLI)
	{
		incrementNumberRespectiveToQuote(lstNewQLI);
	}
	
	private void incrementNumberRespectiveToQuote(List<QuoteLineItem> lstNewQLI)
	{
		Set<Id> setQuoteId = new Set<Id>();
		Map<Id, Integer> mapQuoteIdToMaxNumber = new Map<Id, Integer>();
		
		// gather IDs first of quotes
		for(QuoteLineItem objQLI : lstNewQLI) 
		{
			setQuoteId.add(objQLI.QuoteId);
		}

		if (!setQuoteId.isEmpty()) 
		{
			for (AggregateResult ar : [	SELECT QuoteId, MAX(Item_Numbering__c)
										FROM QuoteLineItem
										GROUP BY QuoteId
										WHERE QuoteId IN: setQuoteId])  
			{
				mapQuoteIdToMaxNumber.put((Id)ar.get('QuoteId'), (Integer)ar.get('expr0'));
			}
			
			for(QuoteLineItem objQLI : lstNewQLI) 
			{
				//  if a QLI is already present for the quote.
				if(mapQuoteIdToMaxNumber.containsKey(objQLI.QuoteId))
				{
					mapQuoteIdToMaxNumber.put(objQLI.QuoteId, mapQuoteIdToMaxNumber.get(objQLI.QuoteId)+1);
				}
				// if this current QLI is the first QLI of the quote.
				else
				{
					mapQuoteIdToMaxNumber.put(objQLI.QuoteId, 1);
				}
				objQLI.Item_Numbering__c = mapQuoteIdToMaxNumber.get(objQLI.QuoteId);
			}
		}
	}
}

trigger QuoteLineItemTrigger on QuoteLineItem (before insert)
{ 
	QuoteLineItemHandler objHandler = new QuoteLineItemHandler();
	
	if(Trigger.isBefore && Trigger.isInsert)
	{
		objHandler.onBeforeInsert(Trigger.New);
	}    
}