3 if you are going to go into the voicemail and use the Line Ownership option. 2 if you are going to use Call Coverage. If you have PC Card Mail, Line Ownership is not even an option.
The difference between Call Coverage and Line Ownership? DND. If the covering/owning extension uses DND, Line Ownership lets the line continue to ring on other extensions before covering to voicemail, while Call Coverage sends the calls immediately to VM when using DND.