If you respect rules stated at the Terms of Service, heading 3. Rules of Conduct, Email Violations, you can send as many as you want:
Email Violations

3.13 Subscribers shall not send bulk email utilizing their resources on the Services unless they maintain a double-authorized list of subscribed members including IP addresses and relevant contact information, along with following guidelines for including removal links with all sent emails according to the such legislation. Subscribers shall comply with all laws regarding the sending of commercial electronic messages or other marketing or electronic communications. Subscribers are forbidden from taking any action that would result in their IP addresses, or any IP address associated with DigitalOcean or other Subscribers, being placed on the Spamhaus.org blacklist. DigitalOcean reserves the sole and absolute right to determine whether an email violation has occurred.