With your current CakeMail plan you can have up to 1000 contacts and send up to 10000 emails per month. This means that if you have 1000 contacts and send email to all of them at one time, you are using 1000 emails of the total of 10000. But if you send it to 600 contacts, you use only 600 mails out of the total. It is possible to check how many are left in the CakeMail account.
If you need a bigger plan with more contacts and larger email limit, you can upgrade your CakeMail plan from your myJOIN portal.