Is there a daily mail merge limit?
1. UNLIMITED EMAILS: If you send hundreds of emails all in one go, Gmail may throw an error at you; this is Gmail's way of trying to discourage spamming. You should also be aware of Gmail Sending Limits.
Therefore, the following workarounds are suggested:
Use tracking so you know which emails have been sent before the error is thrown at you; this way, on the next merge you can continue sending emails from where you left off.
From Google Docs only:
Use scheduling so you spread the emails over time, rather than sending them all in one go.
Send emails over SMTP: You can use third-party SMTP providers (such as SendGrid), use your own SMTP Server or use other email accounts from Yahoo!, Microsoft, etc. You can also send emails over Gmail's SMTP; this would give you an additional 500 emails per day.
2. UNLIMITED LETTERS: When you merge over 100 pages into a single document, you may run into errors as Google Docs are not designed to handle huge content. Hence we suggest you batch your merge.