Are there any limits on the number of email/letter merges that I can perform?
Mail Merge Limits
*Subject to Google-imposed limits (see below).
Google Limits: Emails
If you send hundreds of emails 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:
You can track merged rows so you know which emails have been sent before the error occurs; this way, on the next merge you can continue sending emails where you left off.
From Google Docs only:
You can schedule emails so that you spread them out over time, rather than sending them all in one go.
You can send emails over SMTP using third-party SMTP providers (such as SendGrid), using your own SMTP server or using 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.