If I set up a custom transaction number for my first invoice and then turn it off, will the next invoice follow sequentially?

I'm here to take care of your concern about turning off the Custom Transaction feature, sarah81-manx-net. 

When this feature is disabled, Quickbooks won't assign numbers in sequential order once saved. However, you can enable the Custome Transaction Numbers so that the system will add numbers for your future invoice. to  Let me guide you how:

1. Go to the Gear icon. Under Your Company select Account and Settings. 
2. Select the Sales tab. Locate the Sales form content section.
3. Click on the Pencil icon to edit. 
4. Check the box next to Custom Transaction Numbers.
5. Click the Save button and then Done. 

For additional reference, you can go through these articles to help you in customizing your sales forms: 

• Customize invoices, estimates, and sales receipts
• How to add custom fields to invoices

Please feel free to leave a comment below if you have further concerns about how to maximize and make use of the features in QuickBooks. I'm more than happy to help. Have a good one!