How to create an invoice number automatically for each customer that I have it

Hello, capfranchisesyst.

The default invoice number will always increment the sequence by one. Since it's not incrementing properly, you can reset it by enabling the Custom transaction numbers in the Account and settings page.

I'm happy to show you the steps on how to reset the sequence. 

Here's how: 

1. Go to the Gear icon, then select Account and settings.
2. Click the Sales tab.
3. Open the Sales form content section.
4. Toggle Custom transaction numbers to on. 
5. Click Save then Done. 

After enabling, you'll have the option to enter a new custom number:

The system will automatically increment the numerical value by one. For example if you entered Num-1000 on an invoice, it will show as Num-1001 on the next. Here's a visual demonstration: 

For reference, you can check this article: How to change the invoice number on sales forms. 

Need to run reports for your invoices or want to record other sales transactions? We have a number of articles to help you out. Browse them in our articles page to get started. 

I'll provide guides or pointers again if you have other concerns regarding your invoices or some functions in QuickBooks Online.