The number of ports isn't gonna just depend on the number of VM users. If you are gonna use Unity for your Auto Attendant for all incoming calls, then you have to consider that also. Plus the MWI dial outs. Here's what I found in several Cisco documents:

The Number of Voice Messaging Ports to Install

The number of voice messaging ports to install depends on numerous factors, including:

?The number of calls Cisco Unity will answer when call traffic is at its peak.

?The expected length of each message that callers will record and that subscribers will listen to.

?The number of subscribers.

?The number of ports that will be set to dial out only.

?The number of calls made for message notification.

?The number of MWIs that will be activated when call traffic is at its peak.

?The number of AMIS delivery calls.

?The number of TRAP connections needed when call traffic is at its peak. (TRAP connections are used by Cisco Unity web applications and e-mail clients to play back and record over the phone.)

?The number of calls that will use the automated attendant and call handlers when call traffic is at its peak.

It is best to install only the number of voice messaging ports that are needed so that system resources are not allocated to unused ports.

The Number of Voice Messaging Ports That Will Answer Calls

The calls that the voice messaging ports answer can be incoming calls from unidentified callers or from subscribers. Typically, the voice messaging ports that answer calls are the busiest.

You can set voice messaging ports to both answer calls and to dial out (for example, to send message notifications). However, when the voice messaging ports perform more than one function and are very active (for example, answering many calls), the other functions may be delayed until the voice messaging port is free (for example, message notifications cannot be sent until there are fewer calls to answer). For best performance, dedicate certain voice messaging ports for only answering incoming calls, and dedicate other ports for only dialing out. Separating these port functions eliminates the possibility of a collision, in which an incoming call arrives on a port at the same time that Cisco Unity takes the port off-hook to dial out.

The Number of Voice Messaging Ports That Will Only Dial Out, and Not Answer Calls

Ports that will only dial out and will not answer calls can do one or more of the following:

?Notify subscribers by phone, pager, or e-mail of messages that have arrived.

?Turn MWIs on and off for subscriber extensions.

?Make outbound AMIS calls to deliver voice messages from Cisco Unity subscribers to users on another voice messaging system. (This action is available only with the AMIS licensed feature.)

?Make a TRAP connection so that subscribers can use the phone as a recording and playback device in Cisco Unity web applications and e-mail clients.

Typically, these voice messaging ports are the least busy ports.