One Yola Mail purchase gives you one email address, or mailbox, such as firstname.lastname@example.org. However, you can create as many email aliases for a single mailbox as you'd like.
An email alias is an email address that forwards email to a single mailbox. For instance, you can have "email@example.com", "firstname.lastname@example.org" and email@example.com that all forward to one mailbox.
If you would like to have multiple, separate mailboxes, such as for various people, then you would need to add another mailbox.
You can find information for creating email aliases and adding mailboxes in this tutorial: Setting up your email with Yola Mail.