It is not limited on how many emails you can send but the limit is 10000 recipients per day. So you could send 5000 mails with 2 recipients in each mail or 1000 mails with 10 recipients in each email. There is also a limit of 30 messages per minute.
https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/e...
If you are planning to send out newsletters or similar messages it's recommended to use a third-party tool.
