There are 2 parts of the license- Users and Ports. Users determines how many mailboxes you can support. Ports is how many connections to the phone system you have. They will not be the same!. If you have a 4 port license, 4 people can be talking to unity at any one time. That means leaving messages, using auto attendant, listening to messages, dialing MWI, or Message notification. If I call in to leave a message, as soon as i am transferred to the vm pilot number, I am using the first port. If a user then calls Unity to listen to a message, that is the second port, etc.... until you use 4 ports. The fifth caller to unity gets a busy signal, or transferred to an operator if that is how call manager is configured.
It is hard to determine exactly how many ports are needed for an install, because it will be affected by how much voicemail traffic they have, are they using auto attendant heavily, do they do alot of message notification, etc... But a good rule of thumb is this- Light usage- ports should be 10% of user counts. Heavy usage- Ports should be 20% of user count.
