In the same way that you can only have a limited number of contacts on Mailchimp’s free plan, you can also only send up to a set number of emails, too. Unless you upgrade to the Essentials plan, you won’t be able to send more than 10,000 emails per month.
The Essentials plan allows you to send up to 500,000 emails, the Standard plan up to 1.2 million, and the Premium plan which offers up to 3 million. However, it’s worth bearing in mind that the more emails you can send, the more you’ll need to pay per month.