This is a function of the time you need to send the message in. If you have 1200 minutes (20 hours) then you can do it with just one port. If you need it all 2000 called in 2 minutes then you would need 1000 ports.
The formula for calculating this has many variable depending on the number of rings, number of replays allowed, and other factors that can impact the duration of the call. You can approximate a worst case by adding 24 seconds (4 rings) to 2 time the total duration of both the message and the greeting. This gives you a duration, which you then divide into the time allotted to send the message. This will give you the number of ports needed to send the message to all callers and allows for an answering machine and a replay for each party.