If your item quantity isn't showing up on invoices, it's possible the template you're using has it turned off. In the event this is the case, you'll want to turn it on.
Here's how:
Use your Gear icon, then go to Custom form styles.
Find the template you're using and click its Edit option.
Access your Content tab.
Hit the Table area.
In your COLUMNS section, tick the Quantity checkbox to turn it on.
Select Done.
Now your invoices will show item quantities.
I've also included a detailed resource about customizing invoices which may come in handy moving forward: Customize invoices
Please feel welcome to send a reply if there's any additional questions. Have a a wonderful Wednesday!