What does the error "Limit_Exceeded: Too many target object ids" mean. I couldn't find any documentation related to this. We are getting this error when we do mass emails from Apex in Salesforce.
See the documentation.
You can list up to 250 IDs per email. If you specify a value for the targetObjectIds field, optionally specify a whatId as well to set the email context to a user, contact, or lead. This ensures that merge fields in the template contain the correct data.