currently a budget reseller plan is handling the 80 cpanels account perfectly, my clients web sites are consuming very little bandwidth, and the live streaming is just starting so very few listerners i will have.
Yes, management is a key for VPS too, you have to worry about managing/maintaining your software.
CPanel allows for unlimited domains, however, there is no way to determine exactly how many you can host. There are too many variables such as the amount of hits and traffic, if it's static html, or dynamic php/mysql, etc.
Your best bet is to start with 512MB, keep your eye on the resources and upgrade when needed. The good thing about a VPS is you can upgrade it easily.