Can I keep adding new sales on the same invoice for the same customer or I need to create new ones

It's good to have you here in the Community space, @Eliteking.

To answer your query, yes, you can add new sales items to an existing invoice. However, it is recommended to utilise the Service Date feature to help track the timing of the transaction. 

Let me walk you through the steps on how to enable the feature in QuickBooks Online (QBO):
 

1. In your QBO account, go to the Gear icon, then select Accounts and settings.
2. Select the Sales tab.
3. In the Sales form content section, click on the pencil icon, then enable the Service date.
4. Once ready, click on Save, then Done.
   
   

See the image below for visual reference on the invoice page. Note: The screenshot contains dummy information.


 

On the other hand, if you prefer to track each transaction separately, it would be best to create a new invoice for each sale.

Furthermore, you might also be interested in reading this article that provides guidance on how to record sales transactions in two ways in QuickBooks: Record invoice payments in QuickBooks Online.
 

I'll remain accessible for any other follow-up questions you may have with sales forms. Just tag me in your reply, and I'll respond as quickly as possible.