As was mentioned, QuickBooks Online (QBO) is able to support as many line items on a sales form as are needed. However, it's worth noting, at higher numbers like the one mentioned, you may begin experiencing performance issues.
QBO is only as powerful as the browsers that support it, and they're limited in how much information can be displayed on a single form, regardless of connection speed or hardware specifications. This creates a sort of "soft limit" beyond which you may begin receiving errors or frozen screens. There's an awesome guide on QuickBooks Online's file size and feature limits where you can learn more about this.
Should you begin seeing this performance loss while loading particularly data-heavy pages or forms, these various things will help you keep your QuickBooks running in tip-top shape:
Ensure your web browser is up to date.
Check to see if there are critical performance updates for your operating system.
Run an internet speed test. We recommend using Google Speed Test for checking connection bandwidth.
This information is also available from our guide on Why is my QuickBooks Online company slow? With these instructions, you can keep your QBO running smooth even while loading larger invoices. Please let me know if there's anything else I can do for you, I want to ensure your success. Thanks for reaching out, cheers to a productive week ahead.