How do you define a "mailbox license"? Is that one person?

A mailbox license is an account with a physical email inbox and typically belongs to one person.

Now it is possible to have an account set up for a virtual “role” within your organization that may not be staffed by an individual. So, to decide how many licenses you want to purchase the next question becomes when do you need a new account versus just using an alias or distribution list (which are not considered new accounts).

First some definitions:

Account: “Jane@work.com”, she has a mailbox, calendar, documents, quota, etc.
An account is considered a provisioned account that is counted against your license, regardless of the account status. The only accounts that are not counted against your license are system accounts (for example spam and archiving), and external virtual accounts (for example, a share granted to a user on another email system).
Alias: She can also be “jane.smith@work.com” or even “info@work.com”.
The alias is unique to this account
Jane could receive email sent to info@work.com and would reply back as “jane@work.com”
All the mail sent to the alias lives in her mailbox (if she leaves the mail archive for “info” would be lost)
Distribution List: A group of accounts / aliases
To have a permanent role in your organization for “info@work.com” that doesn’t disappear if Jane moves you will want a new account. This way others can log in to check the email, you will have an archive, you can reply back to messages as “info”, plus Jane won’t have all the “info@work.com” counting against quota.

Important: Microsoft Exchange and Office 365 count disabled or inactive mailboxes against your license, because you are retaining a copy of the data and have access to the account. If you don’t want the mailbox counted against your license, you can retain a copy of the account by exporting the data using your own method or you can delete the account.