Any response here can only be guesswork, as there's so many factors involved - the CPU type is a big part of it, so that'd help greatly, but the first limit you'd hit is probably memory, as a fresh default cPanel can take up about 1GB just to begin with before optimization. A few of our clients have just a single (active) site on a cPanel VPS, which without optimization are usually toeing the 2GB line.
Also depends what kind of sites you'll be aiming at and the price marker. Realistically you could get thousands of accounts onto that setup, doesn't mean you should though.
I will suggest to increase memory to around 3GB for using cPanel and selling hosting.
Number of sites you can host will depend on type of websites / traffic.
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Any dynamic websites will cause little CPU load along with memory usage to the server and it is easy to get an overloaded system if you host a couple of medium traffic websites ( overload for a VPS defined by your provider TOS ). So if you host static websites only, it will be fine to host ~100 websites. Else the number depends on how active the websites will be. Perhaps you can try an nginx / varnish plugin to reduce the load and host more dynamic websites.