How do increase no of person for mailing invoice?

You've come to the right place for mailing your customer's invoices, @customercare2.

To clarify, do you need to send the same invoice to multiple email addresses? If you do, you can add up to three email addresses to the Email field in your customer's profile and separate them by a comma (,). 

Here's how: 

1. Navigate to the Sales menu, then choose Customers.
2. Choose a customer.
3. Select the Customer Details tab, then click Edit.
4. You can enter the email addresses in the Email field.
   
5. Click Save.

Doing this will automatically fill in the customer email field when creating an invoice.

I've added an article for you to know what special characters that aren't acceptable in QuickBooks Online: Acceptable characters in QuickBooks Online.

It'll always be my pleasure to help if you have any other questions. Just keep me posted in the comment section down below. Have a nice day!