So I was wondering: is there a limit?

Yes, the emails are batched to prevent overloading your mail server. You don't want to send too many at once as nearly all hosts have mailer limits to avoid spam and overloading their server. If you're on shared hosting it's best to keep this at 50-200 emails per batch (contact your host for recommended batch size). You can configure how many are sent per batch within CB > Configuration > Mailer then adjust as needed under "Mailer automation settings". Also note some of the emails in your queue may not match your mailer criteria so it's possible some may get discarded when parsed for sending.