If the number of leads exceed your email sending limit of AWS (and MailBluster if you are under the Starter plan), your campaign will not proceed further. The campaign will show Failed. You should keep an eye on your daily sending limit from MailBluster Dashboard before you try to send any email campaigns from MailBluster.
Under Usage, you can see your Free quota usage from Username>My Account. The free quota: 3000 emails are only applicable for a MailBluster account not individual Brand.