Thanks for joining in on this thread. I hope your evening is going well.
When emailing customers from your QuickBooks Online (QBO) account, there's no limit to how many clients you can send an email to. However, there's a chance that your network is getting caught up by the amount of information being sent out.
I recommend selecting as many customers as possible and then sending the email out again. This still allows you to send the email to your clients. You can use the instructions that my colleague provided above or this link about emailing and printing multiple invoices and sales forms.
We're always working hard to improve this financial tool for you and other users. Should you need any further assistance, feel free to ask. The Community is here to have your back. Bye for now!