Since email campaigns are sent through your Gmail account, you are limited to how many Google allows you to send per day. If you are on a paid G Suite account, that limit is generally 2,000/user/day. We further limit you to sending 1,800 campaign emails/user/day to make sure you stay well under that limit.
If you would like to limit this further, you can set an organization-wide daily send limit at Settings > Campaigns:
If you are on a free G Suite account, a new trial account, or on another special package other than a full, paid G Suite account, your email limits are likely much lower. For more information see the G Suite email sending limits.