Each campaign can be sent to a maximum of 200 people.
Important: Please bear in mind that you're sending emails from your personal email account and not a dedicated email marketing tool. If you go over Gmail's send limits or Gmail decides that you're sending spam you could end up having your email account restricted or even blocked completely.
To avoid problems, we recommend following the usual email best practices.
If you send unsolicited emails to dozens of people at a time using Campaigns it is likely that quite a few of those people will mark your emails as spam, and that Google will take anti-spam measures against your account.
Campaigns is a personal emailing tool and isn't a substitute for dedicated email marketing tools that can send email marketing campaigns to 1000's of people.