There is no way you can go over 72 ports on (1) Unity server. The only way you can increase your ports is to add another Unity server. When you add another Unity server it will still only have a max of 72 ports. So what you are basically doing is assigning a (with some logic) users to a Unity server to answer. So for instance.. You could have (4) sets of Unity Primary/Failover each with 72 ports. They would all be assigned to the same Exchange Org (if it's UM).
East Cluster 1 = 72 ports and 7500 subscribers
West Cluster 1 = 72 Ports and 7500 subscrbiers
North Cluster 1 = 72 Ports and 7500 Subscibers
South Cluster 1 = 72 ports and 7500 Subscribers
These would be digitally networked in Unity and Exchange to transfer and foward voicemails, etc. Y
make sense?