Gmail limits the number of emails you can send per day. Rapid MailMerge uses your Gmail account to send email So the limits are applicable to your campaigns as well.
Rapid MailMerge will display your current quota limits on the top right part of the screen. Keep a check on the limits before sending your emails.
No. Rapid MailMerge does attempt to find a way around the Gmail quota limits. (such as scheduling the emailing in batches)
Here is an alternative that you can try: Rapid MailMerge will mark the Email Sent Status column as 'sent' when the email is sent for that row.
You can create a filter where the email status is blank and send the next batch on the next day (after 24 hours).