Are there limits on the number of rows I can merge with Mail Merge add-on?

This article describes the limits associated with the free version of Mail Merge add-on and the limits imposed by Google on Gmail and Google Docs usage for the premium version.

Mail merge limits

*Subject to Google-imposed limits (see below).

Google limits: Gmail

If you send hundreds of emails at once, Gmail may send you an error to discourage spamming.

You should also be aware of Gmail Sending Limits.

Therefore, the following work-arounds are suggested:

Google limits: Letters

When you merge more than 100 pages into a single document, you may receive errors, as Google Docs can only handle a limited amount of content. We suggest that you merge in batches.