How much bandwidth does Ventrilo use? If the amount of people matter, then lets say 10 to 15 people on and in the same channel at the same time.
Ventrilo has several codecs. In your situation, it would be better to have the voice optimize codec running at 24-32 kbps. This is designed for voice only and anything else will get distorted with many artifacts.
If you have the bandwidth, I would suggest that you use 128 kbps/MP3 -- but remember that your voice quality will be affected by your upload bandwidth.