There are many "buzz" on the Web. However, the only official information that I can found is https://support.google.com/mail/answer/22839 . (If you are using Google Apps, the limitation is different)
Several important statements:
Google will temporarily disable your account if you send messages to
more than 500 recipients or if you send a large number of undeliverable messages. ... you may only send a message to 100
people at a time. ... Your account should be re-enabled within 24 hours.
20 emails per hour seems safe. And make sure not sending too many undeliverable emails. The limitation is not based on days but "24 hours".
If you are wondering other email providers/hosting services' limitations, this list may b ehelpful: https://support.mailpoet.com/knowledgebase/lists-of-hosts-and-their-sending-limits/