Currently, we can only send an email promotion per customer at a time in QuickBooks Online. I'd recommend letting our Product Development Team know about this. You can click the Gear icon in the upper-right corner and select Feedback.
As a workaround, you can use your personal or company email provider and send the email promotion in bulk. In addition to that, you might want to export the Customer Contact List report to get all the customer email addresses.
Click Reports on the left menu and search Customer Contact List.
Click the Export option and choose Export to Excel. You can use this report to easily get all the email addresses.
Send the email promotion in bulk using your personal or company email provider.
Feel free to let me know if you need anything else. I'll be around.