What type of integration your best of using depends on several factors like the size of the system, type of PBX and UM Vendor. Generally, Digital integration is faster and more reliable. Analogue is cheaper and you often can start as low as 2 ports per system. The best way of selecting Integration type is not to. Future needs Make sure the system can support your future needs.
Most systems support several thousands of users.
In practice, maximum number of ports generally sets the limit. It's easy enough. Even for me. Personally I prefer to work with a real application if I can. Remember you need to cater for all employees' communication need.